Add PHONON_INCLUDE_PATH to include_directories()

Distributions may have phonon includes in any path. For example, in ArchLinux it is `/usr/include/phonon4qt5/phonon`. It would be nice to add `${PHONON_INCLUDE_PATH}` to `CMakeLists.txt`.
Merge pull request #3 from arcan1s/patch-1

Add PHONON_INCLUDE_PATH to include_directories()