boot: boot_serial: use new Zephyr reboot header

Use the new header used for sys_reboot on Zephyr.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/boot/boot_serial/src/boot_serial.c b/boot/boot_serial/src/boot_serial.c
index d0f0eb0..1d9778d 100644
--- a/boot/boot_serial/src/boot_serial.c
+++ b/boot/boot_serial/src/boot_serial.c
@@ -27,7 +27,7 @@
 #include "bootutil/bootutil_log.h"
 
 #ifdef __ZEPHYR__
-#include <power/reboot.h>
+#include <sys/reboot.h>
 #include <sys/byteorder.h>
 #include <sys/__assert.h>
 #include <drivers/flash.h>