fix the underlinking of lightdm greeter

The lightdm greeter fails to link in the split build mode:

/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/razor-lightdm-greeter.dir/logindata.cpp.o: undefined reference to symbol '_ZN7XdgDirs9cacheHomeEb'
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/../../../../x86_64-pc-linux-gnu/bin/ld: note: '_ZN7XdgDirs9cacheHomeEb' is defined in DSO /usr/lib64/libqtxdg.so.0 so try adding it to the linker command line
/usr/lib64/libqtxdg.so.0: could not read symbols: Invalid operation