Add <string.h> to files that needed it.

diff --git a/msdos/int33.c b/msdos/int33.c
index 7a8ee43..944db4d 100644
--- a/msdos/int33.c
+++ b/msdos/int33.c
@@ -3,6 +3,7 @@
  */
 
 #include <stdlib.h>
+#include <string.h>
 
 #include "windef.h"
 #include "winbase.h"