patch 8.1.0646: cannot build with Ruby 2.6.0

Problem:    Cannot build with Ruby 2.6.0.
Solution:   Add rb_ary_detransient(). (Ozaki Kiichi, closes #3724)
diff --git a/src/version.c b/src/version.c
index 823db35..01593ee 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    646,
+/**/
     645,
 /**/
     644,