clone this repo into your void-packages/srcpkgs folder with the following command git clone https://git.deadzone.lol/Wizzard/mullvad-vpn-void mullvad-vpn (it must be named mullvad-vpn)
After installing this package manually run the command sudo ln -s /opt/MullvadVPN /opt/Mullvad\ VPN
Note that this a temporary work around based on the way Mullvad is packaged