pixdesc: add missing includes

Originally committed as revision 24153 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/pixdesc.c

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