Added missing string.h include.

diff --git a/controls/widgets.c b/controls/widgets.c
index 51c6250..fc409af 100644
--- a/controls/widgets.c
+++ b/controls/widgets.c
@@ -4,6 +4,7 @@
  * Copyright 1993 Alexandre Julliard
  */
 
+#include <string.h>
 #include <assert.h>
 
 #include "win.h"