Makefile.am: Install stable xdg-shell

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
diff --git a/Makefile.am b/Makefile.am
index 0296d5d..cabc279 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,6 +21,7 @@
 stable_protocols =								\
 	stable/presentation-time/presentation-time.xml				\
 	stable/viewporter/viewporter.xml					\
+	stable/xdg-shell/xdg-shell.xml						\
 	$(NULL)
 
 nobase_dist_pkgdata_DATA =							\