Symbian: Phonon MMF: Remove need for mmf_ prefix on source filenames

Some of the header filenames in the Phonon MMF backend are prefixed with mmf_, for example mmf_videoplayer.h. This is to differentiate them from identically named headers in the Phonon library itself. Because of the way in which SYSTEMINCLUDEs and USERINCLUDEs are handled by the Symbian build system, the following code in the Symbian backend

#include "videoplayer.h"

would include the Phonon header, rather than a header from the MMF backend.

Other backends - for example gstreamer - have headers with identical names to Phonon headers, and this does not cause any problems.