include: Define DD_OK to S_OK instead of 0.

Add define for DD_FALSE.
diff --git a/include/ddraw.h b/include/ddraw.h
index 96246e3..f7d1d37 100644
--- a/include/ddraw.h
+++ b/include/ddraw.h
@@ -72,7 +72,8 @@
 #define DDENUMRET_CANCEL	0
 #define DDENUMRET_OK		1
 
-#define DD_OK			0
+#define DD_OK			S_OK
+#define DD_FALSE		S_FALSE
 
 
 #define _FACDD		0x876