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

libavcodec/ituh263enc.c

@@ -38,6 +38,7 @@
 #include "unary.h"
 #include "flv.h"
 #include "mpeg4video.h"
+#include "internal.h"
 
 //#undef NDEBUG
 //#include <assert.h>

libavcodec/mpegvideo_enc.c

@@ -38,6 +38,7 @@
 #include "aandcttab.h"
 #include "flv.h"
 #include "mpeg4video.h"
+#include "internal.h"
 #include <limits.h>
 
 //#undef NDEBUG

libavcodec/svq1enc.c

@@ -31,6 +31,7 @@
 #include "dsputil.h"
 #include "mpegvideo.h"
 #include "h263.h"
+#include "internal.h"
 
 #include "svq1.h"
 #include "svq1enc_cb.h"