Add missing header for av_gcd, fixes the warning:

libaf/af_resample.c:204: warning: implicit declaration of function 'av_gcd'


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@28357 b3059339-0415-0410-9bf9-f77b7e298cf2

libaf/af_resample.c

@@ -25,6 +25,7 @@
 #include <inttypes.h>
 
 #include "libavutil/common.h"
+#include "libavutil/mathematics.h"
 #include "af.h"
 #include "dsp.h"