package/pkg-download.mk: add gitlab macro

package/pkg-download.mk: add gitlab macro

Just like we have a "github" macro to calculate the URL of the tarball
to download source from Github, let's introduce a similar macro for
Gitlab.

This should be used to download the auto-generated tarballs from
Gitlab. If there is a specific release tarball uploaded by the
upstream developers, the <pkg>_SITE variable should not use this new
gitlab macro.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>