Add depends_on 'pkg-config' to hackrf.rb

Tried to install hackrf without having pkg-config installed:

    andres@andress-mbp ~ $ brew install hackrf
    Error: hackrf is a head-only formula
    Install with `brew install --HEAD hackrf`
    andres@andress-mbp ~ $ brew install --HEAD hackrf
    ....
    ==> Installing hackrf
    ==> Cloning https://github.com/mossmann/hackrf.git
    Updating /Library/Caches/Homebrew/hackrf--git
    ==> cmake . -DCMAKE_INSTALL_PREFIX=/usr/local/Cellar/hackrf/HEAD -DCMAKE_BUILD_TYPE=None -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -Wno-dev
    used as include directory in directory /tmp/hackrf-9M77No/host/libhackrf/src
    -- Configuring incomplete, errors occurred!
    See also "/tmp/hackrf-9M77No/host/CMakeFiles/CMakeOutput.log".
    See also "/tmp/hackrf-9M77No/host/CMakeFiles/CMakeError.log".
    READ THIS: https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/Troubleshooting.md#troubleshooting
    If reporting this issue please do so at (not Homebrew/homebrew):
    https://github.com/robotastic/homebrew-hackrf/issues

Installed pkg-config manually:

    andres@andress-mbp ~ $ brew install pkg-config
    ==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/pkg-config-0.28.mavericks.bottle.2.tar.gz
    Already downloaded: /Library/Caches/Homebrew/pkg-config-0.28.mavericks.bottle.2.tar.gz
    ==> Pouring pkg-config-0.28.mavericks.bottle.2.tar.gz
    🍺  /usr/local/Cellar/pkg-config/0.28: 10 files, 604K

After that hacrf builds:

    andres@andress-mbp ~ $ brew install --HEAD hackrf
    ==> Installing hackrf from robotastic/homebrew-hackrf
    ==> Cloning https://github.com/mossmann/hackrf.git
    Updating /Library/Caches/Homebrew/hackrf--git
    ==> cmake . -DCMAKE_INSTALL_PREFIX=/usr/local/Cellar/hackrf/HEAD -DCMAKE_BUILD_TYPE=None -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -Wno-dev
    ==> make install
    🍺  /usr/local/Cellar/hackrf/HEAD: 14 files, 216K, built in 8 seconds