Rename weston_compositor_xkb_init to weston_compositor_set_xkb_rule_names

weston_compositor_xkb_destroy() is called automatically so having only
weston_compositor_xkb_init() to be called by the user was a bit weird.
So rename it so that it makes more sense.
Also export it, since libweston compositors need to call it.

Signed-off-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/src/main.c b/src/main.c
index 475c1d0..39a13cf 100644
--- a/src/main.c
+++ b/src/main.c
@@ -509,7 +509,7 @@
 	weston_config_section_get_string(s, "keymap_options",
 					 (char **) &xkb_names.options, NULL);
 
-	if (weston_compositor_xkb_init(ec, &xkb_names) < 0)
+	if (weston_compositor_set_xkb_rule_names(ec, &xkb_names) < 0)
 		return -1;
 
 	weston_config_section_get_int(s, "repeat-rate",