add initial gitlab-ci.yml

3 jobs for ci in 4 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #236501
xamarin mac
build-libvlcsharp

00:00:30

passed #236502
xamarin mac
build-libvlcsharp-forms

00:00:49

passed #236503
xamarin mac
build-libvlcsharp-samples

00:02:43