patch 8.1.0006: syn_id2cterm_bg() may be undefined

Problem:    syn_id2cterm_bg() may be undefined. (Axel Bender)
Solution:   Adjust #ifdef.
diff --git a/src/version.c b/src/version.c
index a3eb91a..4093a8c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    6,
+/**/
     5,
 /**/
     4,