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/include/sysflash/sysflash.h b/boot/zephyr/include/sysflash/sysflash.h
index 99ca273..95f6cda 100644
--- a/boot/zephyr/include/sysflash/sysflash.h
+++ b/boot/zephyr/include/sysflash/sysflash.h
@@ -3,7 +3,7 @@
 #ifndef __SYSFLASH_H__
 #define __SYSFLASH_H__
 
-#include <devicetree.h>
+#include <zephyr/devicetree.h>
 #include <mcuboot_config/mcuboot_config.h>
 
 #ifndef CONFIG_SINGLE_APPLICATION_SLOT