dhcpcd: fix permissions error with secondary IPv4 addresses

If dhcpcd receives a secondary IPv4 address from the DHCP server it
tries to enable automatic promotion from secondary to primary by writing
`1` to /proc/sys/net/ipv4/conf/%s/promote_secondaries.

Author: rnhmjoj

nixos/modules/services/networking/dhcpcd.nix

@@ -249,7 +249,7 @@ in
             ExecReload = "${dhcpcd}/sbin/dhcpcd --rebind";
             Restart = "always";
             AmbientCapabilities = [ "CAP_NET_ADMIN" "CAP_NET_RAW" "CAP_NET_BIND_SERVICE" ];
-            ReadWritePaths = [ "/proc/sys/net/ipv6" ]
+            ReadWritePaths = [ "/proc/sys/net/ipv4" "/proc/sys/net/ipv6" ]
               ++ lib.optionals useResolvConf ([ "/run/resolvconf" ] ++ config.networking.resolvconf.subscriberFiles);
             DeviceAllow = "";
             LockPersonality = true;