keyboard: fix type of wlr_keyboard_modifiers.group

This isn't a breaking change since both of these typedefs are uint32_t
but this should make things a bit less confusing for readers.

include/wlr/types/wlr_keyboard.h

@@ -45,7 +45,7 @@ struct wlr_keyboard_modifiers {
 	xkb_mod_mask_t depressed;
 	xkb_mod_mask_t latched;
 	xkb_mod_mask_t locked;
-	xkb_mod_mask_t group;
+	xkb_layout_index_t group;
 };
 
 struct wlr_keyboard {

types/wlr_keyboard.c

@@ -48,7 +48,7 @@ bool keyboard_modifier_update(struct wlr_keyboard *keyboard) {
 		XKB_STATE_MODS_LATCHED);
 	xkb_mod_mask_t locked = xkb_state_serialize_mods(keyboard->xkb_state,
 		XKB_STATE_MODS_LOCKED);
-	xkb_mod_mask_t group = xkb_state_serialize_layout(keyboard->xkb_state,
+	xkb_layout_index_t group = xkb_state_serialize_layout(keyboard->xkb_state,
 		XKB_STATE_LAYOUT_EFFECTIVE);
 	if (depressed == keyboard->modifiers.depressed &&
 			latched == keyboard->modifiers.latched &&