iwd.service.in: remove PrivateDevices=true

Previously, the option PrivateDevices=true disabled access to
/dev/rfkill, which lead to:
  'iwctl adapter phy0 set-property Powered {off|on}'
to fail.

This patch explicitly allows access to /dev/rfkill

src/iwd.service.in

@@ -11,7 +11,8 @@ LimitNPROC=1
 Restart=on-failure
 PrivateTmp=true
 NoNewPrivileges=true
-PrivateDevices=true
+DevicePolicy=closed
+DeviceAllow=/dev/rfkill rw
 ProtectHome=yes
 ProtectSystem=strict
 ReadWritePaths=/var/lib/iwd/