Directly #include required X11/XvMC header.

Originally committed as revision 17450 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo_xvmc.c

@@ -20,6 +20,7 @@
  */
 
 #include <limits.h>
+#include <X11/extensions/XvMC.h>
 
 #include "avcodec.h"
 #include "dsputil.h"