patch 8.0.1646: MS-Windows: executable contains unreferenced functions

Problem:    MS-Windows: executable contains unreferenced functions and data.
Solution:   Add /opt:ref to the compiler command. (Ken Takata)
diff --git a/src/version.c b/src/version.c
index 2cc9977..14bf136 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1646,
+/**/
     1645,
 /**/
     1644,