md5: include correct headers

This file needs stdint.h but not string.h
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavutil/md5.c

@@ -30,7 +30,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include <string.h>
+#include <stdint.h>
 #include "bswap.h"
 #include "md5.h"