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/boot/zephyr/os.c b/boot/zephyr/os.c
index 4721eb1..8588a91 100644
--- a/boot/zephyr/os.c
+++ b/boot/zephyr/os.c
@@ -17,7 +17,7 @@
  * under the License.
  */
 
-#include <zephyr.h>
+#include <zephyr/zephyr.h>
 #include <string.h>
 
 #include "os/os_heap.h"