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

diff --git a/dlls/msacm/filter.c b/dlls/msacm/filter.c
index 4085052..40e6d41 100644
--- a/dlls/msacm/filter.c
+++ b/dlls/msacm/filter.c
@@ -6,6 +6,7 @@
  *      Copyright 1998  Patrik Stridvall
  */
 
+#include <string.h>
 #include "winbase.h"
 #include "winnls.h"
 #include "winerror.h"