patch 8.1.0420: generating vim.lib when using ActivePerl 5.20.3 or later

Problem:    Generating vim.lib when using ActivePerl 5.20.3 or later.
Solution:   Redefine XS_EXTERNAL(). (Ken Takata, closes #3462)
diff --git a/src/version.c b/src/version.c
index bc3cbf3..992ed7f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    420,
+/**/
     419,
 /**/
     418,