Add checker for timestamped gzip files

There is an ongoing project to build reproducible deps
(see https://wiki.debian.org/ReproducibleBuilds

). One of tasks
is to update lintian to emit a tag on gzips that contain timestamps.
I've written a simple checker that does exactly that and emits
"package-contains-timestamped-gzip".

Signed-off-by: Bastien ROUCARIÈS <roucaries.bastien+debian@gmail.com>