[gnome-shell] Makefile: Use $(NULL) in one more place

[Jasper St. Pierre <jstpierre src gnome org>]

commit 584f8c83810b9ac691c988d62a49ba5d9240fc04
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Mon Sep 1 13:04:14 2014 -0700

    Makefile: Use $(NULL) in one more place

 src/Makefile.am |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/Makefile.am b/src/Makefile.am
index abffd89..a73eaae 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -101,7 +101,8 @@ shell_public_headers_h =            \
        shell-tray-manager.h            \
        shell-util.h                    \
        shell-window-tracker.h          \
-       shell-wm.h
+       shell-wm.h                      \
+       $(NULL)
 
 if HAVE_NETWORKMANAGER
 shell_public_headers_h +=  shell-network-agent.h