patch 8.0.0631: can't build with Perl 5.26

Problem:    Perl 5.26 also needs S_TOPMARK and S_POPMARK defined.
Solution:   Define the functions when needed. (Jesin, closes #1748)
diff --git a/src/version.c b/src/version.c
index 5bebc54..8f59546 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    631,
+/**/
     630,
 /**/
     629,