Move libgitlab-api-v4-perl to Suggests on Ubuntu

Use the logic present in debian/rules to add libgitlab-api-v4-perl to Suggests on Ubuntu and keep it in Recommends on Debian.

The reason for this change is that we're considering moving libgitlab-api-v4-perl to universe on Ubuntu but devscripts is in main.

When libgitlab-api-v4-perl is not installed a message will be displayed:

# salsa
You must install GitLab::API::v4