patch 8.1.0951: using WIN64 even though it is never defined

Problem:    Using WIN64 even though it is never defined.
Solution:   Only use _WIN64. (Ken Takata, closes #3997)

src/version.c

@@ -779,6 +779,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    951,
 /**/
     950,
 /**/