Fix implicit declaration warning.

Originally committed as revision 21114 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/internal.h

@@ -42,4 +42,6 @@ int ff_is_hwaccel_pix_fmt(enum PixelFormat pix_fmt);
  */
 AVHWAccel *ff_find_hwaccel(enum CodecID codec_id, enum PixelFormat pix_fmt);
 
+int ff_match_2uint16(const uint16_t (*tab)[2], int size, int a, int b);
+
 #endif /* AVCODEC_INTERNAL_H */