md5.c: Always include string.h to fix memcpy warnings

src/md5.c

@@ -39,6 +39,7 @@
  */
 
 #include "md5.h"
+#include <string.h>
 
 #ifdef TEST
 /*
@@ -46,7 +47,6 @@
  * The test program should print out the same values as given in section
  * A.5 of RFC 1321, reproduced below.
  */
-#include <string.h>
 main()
 {
 	static const char *const test[7] = {