Prefer the systemd-dev package as a build dependency
When building, we only need the systemd.pc pkgconfig file, and systemd-dev is a new package which, if available, contains that file without bringing in the whole of systemd. So prefer using that package as a build dependency.
* debian/control (Source: mandos/Build-Depends): Change "systemd" to "systemd-dev | systemd".
Closes: #1060546 Reported-By: Michael Biebl <biebl@debian.org>