Add missing internal.h to files calling ff_match_2uint16().

Fixes warnings:
libavcodec/mpegvideo_enc.c:574: warning: implicit declaration of function
'ff_match_2uint16'
libavcodec/ituh263enc.c:143: warning: implicit declaration of function
'ff_match_2uint16'
libavcodec/svq1enc.c:97: warning: implicit declaration of function
'ff_match_2uint16'

Originally committed as revision 21133 to svn://svn.ffmpeg.org/ffmpeg/trunk