[0.9.22] fix compilation with dmd master

Similar to 022102598253dcba8be38bcf74ad3cb7e7d030ee

NB: this is PR against 0.9.22 branch. I need it merged to make stable dub version compatible with dmd git master to not screw my CI system (https://jenkins.dicebot.lv)

Making point release for 0.9.22 with this merged is not strictly necessary as it does not affect any functionality