input: Move the xkb fields from weston_seat to weston_keyboard

XKB integration is a keyboard feature and should as such be kept track
of in the keyboard struct.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
diff --git a/src/compositor-wayland.c b/src/compositor-wayland.c
index 7b280b8..14ff4af 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -1270,7 +1270,7 @@
 	else
 		serial_out = wl_display_next_serial(c->base.wl_display);
 
-	xkb_state_update_mask(input->base.xkb_state.state,
+	xkb_state_update_mask(input->base.keyboard->xkb_state.state,
 			      mods_depressed, mods_latched,
 			      mods_locked, 0, 0, group);
 	notify_modifiers(&input->base, serial_out);