patch 8.1.0931: vtp_working included in GUI build but unused

Problem:    vtp_working included in GUI build but unused.
Solution:   Adjust #ifdefs. (Ken Takata, closes #3971)
diff --git a/src/version.c b/src/version.c
index e2b6f7a..00c63e5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -784,6 +784,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    931,
+/**/
     930,
 /**/
     929,