app: use <kernel.h> include

<zephyr.h> is a proxy to <kernel.h> in practice, so let's use <kernel.h>
instead.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

Author: gmarull

app/src/main.c

@@ -3,7 +3,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr.h>
+#include <kernel.h>
 #include <drivers/sensor.h>
 
 #include "app_version.h"