Remove bogus forward declarations

Remove forward declarations of MediaBuffer -- they're not needed because
MediaBuffer.h is included earlier in the file anyway, and they
incorrectly declared MediaBuffer as a struct instead of a class.

Change-Id: Ia35ea8f938553e39244032b6fd6c061aa388859f
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>