support reboot -f for kexec kernel

Fix error message:

-->--
Code should not be reached 'Unknown action.' at
src/systemctl/systemctl.c:6382, function halt_now(). Aborting.
Aborted
--<--

when executing 'reboot -f' from a system running a kexec kernel.

src/systemctl/systemctl.c

@@ -7292,6 +7292,7 @@ static int halt_now(enum action a) {
                 reboot(RB_POWER_OFF);
                 return -errno;
 
+        case ACTION_KEXEC:
         case ACTION_REBOOT: {
                 _cleanup_free_ char *param = NULL;