refactor: Move to zephyr/ include paths.

* Zephyr moved to properly namespaced headers, so major "rip
  the bandaid" commit to move us to those everywhere.

app/src/events/battery_state_changed.c

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: MIT
  */
 
-#include <kernel.h>
+#include <zephyr/kernel.h>
 #include <zmk/events/battery_state_changed.h>
 
 ZMK_EVENT_IMPL(zmk_battery_state_changed);