Prevent multiple definitions caused by MSVCRT headers.

diff --git a/include/winnt.h b/include/winnt.h
index fb430e1..44459a3 100644
--- a/include/winnt.h
+++ b/include/winnt.h
@@ -12,6 +12,7 @@
 #ifndef RC_INVOKED
 #include <ctype.h>
 #include <stddef.h>
+#include <string.h>
 #endif
 
 
@@ -250,11 +251,14 @@
 typedef long            LONG,       *PLONG,    *LPLONG;
 
 /* Some systems might have wchar_t, but we really need 16 bit characters */
+#ifndef WINE_WCHAR_DEFINED
 #ifdef WINE_UNICODE_NATIVE
 typedef wchar_t         WCHAR,      *PWCHAR;
 #else
 typedef unsigned short  WCHAR,      *PWCHAR;
 #endif
+#define WINE_WCHAR_DEFINED
+#endif
 
 /* 'Extended/Wide' numerical types */
 #ifndef _ULONGLONG_