libweston/launcher: annotate iface(s) as constant data

Already considered and handled as such.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/libweston/launcher-impl.h b/libweston/launcher-impl.h
index ba2cd8e..404383a 100644
--- a/libweston/launcher-impl.h
+++ b/libweston/launcher-impl.h
@@ -42,9 +42,9 @@
 };
 
 struct weston_launcher {
-	struct launcher_interface *iface;
+	const struct launcher_interface *iface;
 };
 
-extern struct launcher_interface launcher_logind_iface;
-extern struct launcher_interface launcher_weston_launch_iface;
-extern struct launcher_interface launcher_direct_iface;
+extern const struct launcher_interface launcher_logind_iface;
+extern const struct launcher_interface launcher_weston_launch_iface;
+extern const struct launcher_interface launcher_direct_iface;