patch 8.2.0646: t_Co uses the value of $COLORS in the GUI

Problem:    t_Co uses the value of $COLORS in the GUI. (Masato Nishihata)
Solution:   Ignore $COLORS for the GUI. (closes #5992)
diff --git a/src/version.c b/src/version.c
index 392ac18..16145f2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    646,
+/**/
     645,
 /**/
     644,