nixos/dhcpcd: restart dhcpcd if exit hook changed

This change ensures that `dhcpcd.service` is restarted as soon as the
exit hook changes. I use this hook to do additional configuration for my
network (like setting a route via the given gateway to my WireGuard) and
when changing parts of this exit hook I'd expect to get this activated
when switching to my new configuration.

nixos/modules/services/networking/dhcpcd.nix

@@ -165,6 +165,8 @@ in
         before = [ "network-online.target" ];
         after = [ "systemd-udev-settle.service" ];
 
+        restartTriggers = [ exitHook ];
+
         # Stopping dhcpcd during a reconfiguration is undesirable
         # because it brings down the network interfaces configured by
         # dhcpcd.  So do a "systemctl restart" instead.