removed duplicate forward declaration for VFS in engine.h. Also added a forward declaration for SoundClipPool in engine.h because it has a pointer to a SoundClipPool.
removed duplicate forward declaration for VFS in engine.h. Also added a forward declaration for SoundClipPool in engine.h because it has a pointer to a SoundClipPool.

fifengine/fifengine,cbeck88/fifengine,cbeck88/fifengine,gravitystorm/fifengine,gravitystorm/fifengine,gravitystorm/fifengine,fifengine/fifengine,gravitystorm/fifengine,cbeck88/fifengine,Niektory/fifengine,Niektory/fifengine,cbeck88/fifengine,Niektory/fifengine,fifengine/fifengine