Include string.h to make winnt.h self-sufficient.

diff --git a/include/winnt.h b/include/winnt.h
index adfc56d..dc2877a 100644
--- a/include/winnt.h
+++ b/include/winnt.h
@@ -9,6 +9,10 @@
 
 #include "windef.h"
 
+#ifndef RC_INVOKED
+#include <string.h>
+#endif
+
 #include "pshpack1.h"
 /* Defines */