link -lrt to osrm-prepare

Avoids linking error for me on Ubuntu precise:

```sh
Building CXX object CMakeFiles/osrm-routed.dir/Server/RequestHandler.cpp.o
/usr/bin/g++-4.8    -std=c++11 -DBOOST_SPIRIT_USE_PHOENIX_V3=1 -I/home/ubuntu/mapnik-packaging/osx/out/build-cpp11-libstdcpp-gcc-x86_64-linux/include  -DNDEBUG -O3  -D_FILE_OFFSET_BITS=64 -fPIC  -Wall -pedantic -fPIC            -flto -O3 -DNDEBUG -I/home/ubuntu/mapnik-packaging/osx/out/packages/Project-OSRM/Include -I/home/ubuntu/mapnik-packaging/osx/out/build-cpp11-libstdcpp-gcc-x86_64-linux/include -I/home/ubuntu/mapnik-packaging/osx/out/build-cpp11-libstdcpp-gcc-x86_64-linux/include/libxml2 -I/home/ubuntu/mapnik-packaging/osx/out/build-cpp11-libstdcpp-gcc-x86_64-linux/include/luabind -I/home/ubuntu/mapnik-packaging/osx/out/build-cpp11-libstdcpp-gcc-x86_64-linux/include/osmpbf    -std=c++11 -o CMakeFiles/osrm-routed.dir/Server/RequestHandler.cpp.o -c /home/ubuntu/mapnik-packaging/osx/out/packages/Project-OSRM/Server/RequestHandler.cpp
/home/ubuntu/mapnik-packaging/osx/out/build-cpp11-libstdcpp-gcc-x86_64-linux/lib/libboost_thread.a(thread.o): In function `boost::this_thread::hiden::sleep_for(timespec const&)':
thread.cpp:(.text+0x132e): undefined reference to `clock_gettime'
/home/ubuntu/mapnik-packaging/osx/out/build-cpp11-libstdcpp-gcc-x86_64-linux/lib/libboost_thread.a(thread.o): In function `boost::this_thread::hiden::sleep_until(timespec const&)':
thread.cpp:(.text+0x1495): undefined reference to `clock_gettime'
thread.cpp:(.text+0x1518): undefined reference to `clock_gettime'
collect2: error: ld returned 1 exit status
make[2]: *** [osrm-prepare] Error 1
make[2]: Leaving directory `/home/ubuntu/mapnik-packaging/osx/out/packages/Project-OSRM/build'
make[1]: *** [CMakeFiles/osrm-prepare.dir/all] Error 2
```