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.
diff --git a/src/version.c b/src/version.c
index f36b986..4a6fb00 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1538,
+/**/
     1537,
 /**/
     1536,