mullvad: make openvpn support optional

Enabled by default to not break anybody who depends on it, but I don't
and I would like to disable it.

pkgs/applications/networking/mullvad/mullvad.nix

@@ -12,6 +12,7 @@
 , libnftnl
 , libmnl
 , libwg
+, enableOpenvpn ? true
 , openvpn-mullvad
 , shadowsocks-rust
 }:
@@ -67,7 +68,7 @@ rustPlatform.buildRustPackage rec {
       done
     '' +
     # Files necessary for OpenVPN tunnels to work.
-    ''
+    lib.optionalString enableOpenvpn ''
       mkdir -p $out/share/mullvad
       cp dist-assets/ca.crt $out/share/mullvad
       ln -s ${openvpn-mullvad}/bin/openvpn $out/share/mullvad