patch 8.1.2006: build failure with huge features but without channel feature

Problem:    Build failure with huge features but without channel feature.
Solution:   Add #ifdef. (Dominique Pelle, closes #4906)
diff --git a/src/version.c b/src/version.c
index dd0fe62..4f9e346 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2006,
+/**/
     2005,
 /**/
     2004,