zephyr: use <zephyr/kernel.h>

<zephyr/zephyr.h> is just a shim to the <zephyr/kernel.h> header, better
use it directly as it is explicit that Kernel APIs are being included.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index c902978..d9e39c1 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -5,7 +5,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include <zephyr/devicetree.h>
 #include <zephyr/drivers/flash.h>