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/samples/zephyr/hello-world/src/main.c b/samples/zephyr/hello-world/src/main.c
index 584a21c..66443d7 100644
--- a/samples/zephyr/hello-world/src/main.c
+++ b/samples/zephyr/hello-world/src/main.c
@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include <zephyr/sys/printk.h>
 
 void main(void)