ffmetaenc: Use correct format specifiers.

Use printf format macros from inttypes.h.
Additionally, this fixes a warning when building with clang.
(cherry picked from commit c0ae5152d1)

libavformat/ffmetaenc.c

@@ -19,6 +19,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <inttypes.h>
+
 #include "avformat.h"
 #include "ffmeta.h"