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-direct.c b/libweston/launcher-direct.c
index 4fc1c3c..4e878a5 100644
--- a/libweston/launcher-direct.c
+++ b/libweston/launcher-direct.c
@@ -319,7 +319,7 @@
 	return minor(s.st_rdev);
 }
 
-struct launcher_interface launcher_direct_iface = {
+const struct launcher_interface launcher_direct_iface = {
 	launcher_direct_connect,
 	launcher_direct_destroy,
 	launcher_direct_open,