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)
diff --git a/src/version.c b/src/version.c
index fe498ec..ee34042 100644
--- a/src/version.c
+++ b/src/version.c
@@ -780,6 +780,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    951,
+/**/
     950,
 /**/
     949,