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/serial_adapter.c b/boot/zephyr/serial_adapter.c
index 4934f6d..309d72a 100644
--- a/boot/zephyr/serial_adapter.c
+++ b/boot/zephyr/serial_adapter.c
@@ -18,7 +18,7 @@
 #include <zephyr/drivers/uart.h>
 #include <assert.h>
 #include <string.h>
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include "bootutil/bootutil_log.h"
 #include <zephyr/usb/usb_device.h>