nixos/matrix-synapse: reload service with SIGHUP

This is used to load new certificates without restarting the service

nixos/modules/services/misc/matrix-synapse.nix

@@ -695,6 +695,7 @@ in {
             ${ concatMapStringsSep "\n  " (x: "--config-path ${x} \\") ([ configFile ] ++ cfg.extraConfigFiles) }
             --keys-directory ${cfg.dataDir}
         '';
+        ExecReload = "${pkgs.utillinux}/bin/kill -HUP $MAINPID";
         Restart = "on-failure";
       };
     };