examples/filtering_video: add missing headers

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/examples/filtering_video.c

@@ -29,6 +29,8 @@
 
 #define _XOPEN_SOURCE 600 /* for usleep */
 #include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
 
 #include <libavcodec/avcodec.h>
 #include <libavformat/avformat.h>