xwayland: Staticize weston_wm_set_selection and read_and_dump_property

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>
diff --git a/src/xwayland/selection.c b/src/xwayland/selection.c
index cd49289..edc7ce6 100644
--- a/src/xwayland/selection.c
+++ b/src/xwayland/selection.c
@@ -616,7 +616,7 @@
 	return 0;
 }
 
-void
+static void
 weston_wm_set_selection(struct wl_listener *listener, void *data)
 {
 	struct wl_seat *seat = data;