Fixed a few broken includes.

diff --git a/if1632/builtin.c b/if1632/builtin.c
index a514170..a817cc8 100644
--- a/if1632/builtin.c
+++ b/if1632/builtin.c
@@ -9,6 +9,7 @@
 #include <string.h>
 #include "winbase.h"
 #include "wine/winbase16.h"
+#include "wine/winestring.h"
 #include "builtin32.h"
 #include "global.h"
 #include "heap.h"