MT#7327 Debian packaging: depend on libfile-spec-perl or recent perl-base version

Same as for Build-Depends we need to do the same for Depends (c/p from git commit 59819ab11c7235):

Perl 5.20 provides the module which was shipped via libfile-spec-perl so far:
https://packages.qa.debian.org/libf/libfile-spec-perl/news/20140927T163912Z.html

So either depend on libfile-spec-perl OR an according fresh perl-base package.