nixos/dhcpcd: Only run resume commands if enabled.

The networkd implementation sets systemd.services.dhcpcd.enable to
false in nixos/modules/tasks/network-interfaces-systemd.nix. So we need
to respect that in the dhcpcd module.

If we don't, the resumeCommand is set nevertheless, which causes the
post-resume.service to fail after resuming:

Failed to reload dhcpcd.service: Unit dhcpcd.service is masked.
post-resume.service: main process exited, code=exited, status=1/FAILURE
Failed to start Post-Resume Actions.
Dependency failed for Post-Resume Actions.
Unit post-resume.service entered failed state.
post-resume.service failed.

Signed-off-by: aszlig <[e-mail]>

nixos/modules/services/networking/dhcpcd.nix

···
         }
       ];
 
-    powerManagement.resumeCommands =
+    powerManagement.resumeCommands = mkIf config.systemd.services.dhcpcd.enable
       ''
         # Tell dhcpcd to rebind its interfaces if it's running.
         ${config.systemd.package}/bin/systemctl reload dhcpcd.service