libweston: constify data_source_interface

Found mutable global variables with
 $ grep -P '^static (?!const).*[=;]' -- compositor libweston shared

Mutable global or static variables make it harder to run several
compositor instances in the same process. That is what the test suite
would probably need to do to test wayland-backend.

This one variable does not need to be mutable.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/data-device.c

@@ -401,7 +401,7 @@ data_source_set_actions(struct wl_client *client,
 	source->actions_set = true;
 }
 
-static struct wl_data_source_interface data_source_interface = {
+static const struct wl_data_source_interface data_source_interface = {
 	data_source_offer,
 	data_source_destroy,
 	data_source_set_actions