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

dlls/comctl32/monthcal.c

diff --git a/dlls/comctl32/monthcal.c b/dlls/comctl32/monthcal.c
index 0e0d519..0ba750b 100644
--- a/dlls/comctl32/monthcal.c
+++ b/dlls/comctl32/monthcal.c
@@ -19,6 +19,7 @@
 #include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include "winbase.h"
 #include "windef.h"