Bug#853619: proposed fix

[Kir Kolyshkin]

A fix has been committed to ploop repo:

https://src.openvz.org/projects/OVZL/repos/ploop/commits/d42955558fb98245be22ebe9f576e455c7bf9b26

As gcc-7 is still experimental/unreleased, no new ploop release
is currently planned.