mpeg12enc: Add missing #include for PICT_FRAME

(cherry picked from commit 8fc6a70c21)
Signed-off-by: Diego Biurrun <diego@biurrun.de>

libavcodec/mpeg12enc.c

@@ -37,6 +37,7 @@
 #include "mathops.h"
 #include "mpeg12.h"
 #include "mpeg12data.h"
+#include "mpegutils.h"
 #include "mpegvideo.h"