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/os.c b/boot/zephyr/os.c
index 8588a91..048cb2b 100644
--- a/boot/zephyr/os.c
+++ b/boot/zephyr/os.c
@@ -17,7 +17,7 @@
  * under the License.
  */
 
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include <string.h>
 
 #include "os/os_heap.h"