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/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index c5832cb..c902978 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -5,9 +5,9 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr.h>
-#include <devicetree.h>
-#include <drivers/flash.h>
+#include <zephyr/zephyr.h>
+#include <zephyr/devicetree.h>
+#include <zephyr/drivers/flash.h>
 
 #include "target.h"