qgis: added dependency on libspatialite

I couldn't compile without it. After a manual `brew install libspatialite` the `qgis` formula worked seamlessly.

The error I was getting was:

    CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
    Please set them or make sure they are set and tested correctly in the CMake files:
    SPATIALITE_INCLUDE_DIR
       used as include directory in directory /tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/src/plugins/offline_editing
    SPATIALITE_LIBRARY
        linked by target "offlineeditingplugin" in directory /tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/src/plugins/offline_editing

The full output was:

    $ brew install qgis
    Warning: You have Xcode-4.3.2, which is outdated.
    Please install Xcode 4.3.3.
    ==> Downloading http://qgis.org/downloads/qgis-1.8.0.tar.bz2
    Already downloaded: /Users/chris.berkhout/Library/Caches/Homebrew/qgis-1.8.0.tar.bz2
    ==> Downloading http://ftpmirror.gnu.org/bison/bison-2.5.tar.bz2
    Already downloaded: /Users/chris.berkhout/Library/Caches/Homebrew/bison-2.5.tar.bz2
    ==> ./configure --prefix=/private/tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/bison --disable-debug
    ==> make install
    ==> Downloading http://sourceforge.net/projects/qwt/files/qwt/5.2.2/qwt-5.2.2.tar.bz2
    Already downloaded: /Users/chris.berkhout/Library/Caches/Homebrew/qwt-5.2.2.tar.bz2
    ==> qmake -spec macx-g++ -config release
    ==> make install
    ==> cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local/Cellar/qgis/1.8.0 -DCMAKE_BUILD_TYPE=None -DCMAKE_FIND_FRAMEWORK=LA
    -- The C compiler identification is GNU 4.2.1
    -- The CXX compiler identification is GNU 4.2.1
    -- Checking whether C compiler has -isysroot
    -- Checking whether C compiler has -isysroot - yes
    -- Checking whether C compiler supports OSX deployment target flag
    -- Checking whether C compiler supports OSX deployment target flag - yes
    -- Check for working C compiler: /usr/bin/llvm-gcc
    -- Check for working C compiler: /usr/bin/llvm-gcc -- works
    -- Detecting C compiler ABI info
    -- Detecting C compiler ABI info - done
    -- Checking whether CXX compiler has -isysroot
    -- Checking whether CXX compiler has -isysroot - yes
    -- Checking whether CXX compiler supports OSX deployment target flag
    -- Checking whether CXX compiler supports OSX deployment target flag - yes
    -- Check for working CXX compiler: /usr/bin/llvm-g++
    -- Check for working CXX compiler: /usr/bin/llvm-g++ -- works
    -- Detecting CXX compiler ABI info
    -- Detecting CXX compiler ABI info - done
    -- Quantum GIS version: 1.8.0 Lisboa (10800)
    -- Could not find GRASS
    -- Touch support disabled
    -- Looking for openpty
    -- Looking for openpty - found
    -- Found Proj: /usr/local/lib/libproj.dylib
    -- Found GEOS: /usr/local/Cellar/geos/3.3.5/lib/libgeos_c.dylib
    -- Found GDAL: /usr/local/Cellar/gdal/1.9.1/lib/libgdal.dylib (1.9.1)
    -- Found Expat: /usr/lib/libexpat.dylib
    -- Found Spatialindex: /usr/local/lib/libspatialindex.dylib
    -- Found Qwt: /tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/qwt52/lib/libqwt.a (5.2.2)
    -- Found Sqlite3: /usr/local/lib/libsqlite3.dylib
    -- Found PostgreSQL: /usr/lib/libpq.dylib
    -- Looking for Q_WS_X11
    -- Looking for Q_WS_X11 - not found.
    -- Looking for Q_WS_WIN
    -- Looking for Q_WS_WIN - not found.
    -- Looking for Q_WS_QWS
    -- Looking for Q_WS_QWS - not found.
    -- Looking for Q_WS_MAC
    -- Looking for Q_WS_MAC - found
    -- Looking for QT_MAC_USE_COCOA
    -- Looking for QT_MAC_USE_COCOA - found
    -- Found Qt4: /usr/local/bin/qmake (found suitable version "4.8.2", required is "4.4.0") 
    -- Pedantic compiler settings enabled
    -- Found PythonInterp: /usr/bin/python (found version "2.7.1") 
    -- Found Python executable: /usr/bin/python
    -- Found Python version: 2.7.1
    -- Found Python library: -framework Python
    -- Found SIP version: 4.13.3
    -- Found PyQt4 version: 4.9.4
    -- Found GSL: -L/usr/local/Cellar/gsl/1.15/lib -lgsl -lgslcblas
    -- txt2tags not found - disabled
    CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
    Please set them or make sure they are set and tested correctly in the CMake files:
    SPATIALITE_INCLUDE_DIR
       used as include directory in directory /tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/src/plugins/offline_editing
    SPATIALITE_LIBRARY
        linked by target "offlineeditingplugin" in directory /tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/src/plugins/offline_editing

    -- Configuring incomplete, errors occurred!
    ==> Build Environment
    HOMEBREW_VERSION: 0.9.2
    HEAD: a732c9b9913b4e2fc5a85ef9261cc80ec84276b5
    CPU: dual-core 64-bit penryn
    OS X: 10.7.3-x86_64
    Xcode: 4.3.2
    CLT: 1.0.0.9000000000.1.1249367152
    GCC-4.0: N/A
    GCC-4.2: build 5666
    LLVM: build 2335
    Clang: 2.1 build 163
    X11: 2.6.3 @ /usr/X11
    CC: /usr/bin/llvm-gcc => /usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
    CXX: /usr/bin/llvm-g++ => /usr/llvm-gcc-4.2/bin/llvm-g++-4.2
    LD: /usr/bin/llvm-gcc => /usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
    CFLAGS: -Os -w -pipe -march=core2 -msse4.1 -mmacosx-version-min=10.7
    CXXFLAGS: -Os -w -pipe -march=core2 -msse4.1 -mmacosx-version-min=10.7
    CPPFLAGS: -I/usr/local/Cellar/readline/6.2.4/include -isystem /usr/local/include
    LDFLAGS: -L/usr/local/Cellar/readline/6.2.4/lib -L/usr/local/lib
    MACOSX_DEPLOYMENT_TARGET: 10.7
    MAKEFLAGS: -j2
    Error: Failed executing: cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local/Cellar/qgis/1.8.0 -DCMAKE_BUILD_TYPE=None -DCMAKE_FIND_FRAMEWORK=LAST -Wno-dev -DQWT_INCLUDE_DIR=/private/tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/qwt52/include -DQWT_LIBRARY=/private/tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/qwt52/lib/libqwt.a -DBISON_EXECUTABLE=/private/tmp/homebrew-qgis-1.8.0-1g5s/qgis-1.8.0/bison/bin/bison -DENABLE_TESTS=NO -DQGIS_MACAPP_BUNDLE=0 -DQGIS_MACAPP_DEV_PREFIX='/usr/local/Cellar/qgis/1.8.0/lib' -DQGIS_MACAPP_INSTALL_DEV=YES (qgis.rb:108)
    This may help you fix or report the issue if `brew doctor` does not:
        https://github.com/homebrew/homebrew-science/issues/new
    $