nixos/redmine: Add PrivateMounts to systemd unit settings

PrivateMounts seems to be enabled anyway. It's probably enabled by
another setting. However, in order to not create a wrong impression, add
it to the list and enable it.

Signed-off-by: Felix Singer <felixsinger@posteo.net>

nixos/modules/services/misc/redmine.nix

@@ -442,6 +442,7 @@ in
         MemoryDenyWriteExecute = true;
         NoNewPrivileges = true;
         PrivateDevices = true;
+        PrivateMounts = true;
         PrivateTmp = true;
         ProcSubset = "pid";
         ProtectClock = true;