zephyr: migrate includes to <zephyr/...>

Zephyr has prefixed all of its includes with <zephyr/...>. While the
_old_ mode can still be used (CONFIG_LEGACY_INCLUDE_PATH) and is still
enabled by default, it's better to be prepared for its removal in the
future.

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 027bb5f..584a21c 100644
--- a/samples/zephyr/hello-world/src/main.c
+++ b/samples/zephyr/hello-world/src/main.c
@@ -4,8 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr.h>
-#include <sys/printk.h>
+#include <zephyr/zephyr.h>
+#include <zephyr/sys/printk.h>
 
 void main(void)
 {