Add gitlab.com into bsd.sites.mk

[L.Bartoletti]

Hello,

I would suggest to add gitlab.com into bsd.sites.mk

Since, I think I am not competent to propose a patch, I ask for help.

I have ports to create where the sources are only available on GitLab. 
Like some other ports (audio/midi-matrix-lv2, audio/moony-lv2, etc.) I 
can fetch source using a direct link. But, it may be nice to have this 
possibility.

As I can see, it can be similar (and maybe easier) than github.

Archive can be dowload by this URL:
https://gitlab.com/${GL_ACCOUNT}/${GL_PROJECT}/repository/${GL_TAGNAME}/archive.${EXTRACT_SUFX}

${EXTRACT_SUFX} can be zip, tar.gz, tar.bz2 and tar

Is anyone already working on it or do you want to do it with me?

Thanks.

Regards.