patch 8.1.0939: no completion for sign group names

Problem:    No completion for sign group names.
Solution:   Add completion for sign group names and buffer names. (Yegappan
            Lakshmanan, closes #3980)
diff --git a/src/version.c b/src/version.c
index df3aa48..06500bd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -780,6 +780,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    939,
+/**/
     938,
 /**/
     937,