Remove redundant av_log2_16bit forward declaration, fixes checkheaders warning:

libavutil/internal.h:167: warning: redundant redeclaration of `av_log2_16bit'
libavutil/common.h:143: warning: previous definition of `av_log2_16bit' was here

Originally committed as revision 16773 to svn://svn.ffmpeg.org/ffmpeg/trunk