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/hooks_sample.c b/boot/zephyr/hooks_sample.c
index b151d0d..baa2b75 100644
--- a/boot/zephyr/hooks_sample.c
+++ b/boot/zephyr/hooks_sample.c
@@ -15,7 +15,7 @@
  */
 
 #include <assert.h>
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include "bootutil/image.h"
 #include "bootutil/bootutil.h"
 #include "bootutil/fault_injection_hardening.h"