libxvid: add missing include of libavutil/mathematics.h

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/libxvidff.c

@@ -30,6 +30,7 @@
 #include "avcodec.h"
 #include "libavutil/cpu.h"
 #include "libavutil/intreadwrite.h"
+#include "libavutil/mathematics.h"
 #include "libxvid_internal.h"
 #if !HAVE_MKSTEMP
 #include <fcntl.h>