patch 8.0.0163: cannot build with Ruby 2.4

Problem:    Ruby 2.4 no longer supports rb_cFixnum.
Solution:   move rb_cFixnum into an #ifdef. (Kazuki Sakamoto, closes #1365)
diff --git a/src/version.c b/src/version.c
index 1c4c57b..9b0809b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    163,
+/**/
     162,
 /**/
     161,