Set Standards-Version in debian/control depending on the debian release

--- » cat bitmask-dev/debian/control 
Source: bitmask
Section: misc
Priority: extra
Maintainer: Kali Kaneko <kali@leap.se>
Uploaders: LEAP automated build <sysdev@leap.se>
Homepage: https://bitmask.net
Standards-Version: 3.9.8
…

This works with stretch, but buster lintian gives out a warning because a newer standards version is available (4.0.0).

We can't substitute variables in the Source: section as we can do in the Package: section, therefore we need to have a better way to solve this (see this related discussion).