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/hooks_sample.c b/boot/zephyr/hooks_sample.c
index a4dbfb5..b151d0d 100644
--- a/boot/zephyr/hooks_sample.c
+++ b/boot/zephyr/hooks_sample.c
@@ -15,7 +15,7 @@
  */
 
 #include <assert.h>
-#include <zephyr.h>
+#include <zephyr/zephyr.h>
 #include "bootutil/image.h"
 #include "bootutil/bootutil.h"
 #include "bootutil/fault_injection_hardening.h"