patch 8.1.1538: cannot specify highlighting for notifications

Problem:    Cannot specify highlighting for notifications.
Solution:   Use the PopupNotification group if it exists. Add a minimal width
            to notifications.

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1538,
 /**/
     1537,
 /**/