gitlab-ci: do not wait for all jobs from "dependencies" stage.

The GNU/Linux builds should start as soon as the Linux dependencies are
built. There is no need to wait for the Windows dependencies (and
reciprocally of course).
This should make for much faster CI total duration (with current
configuration).

Note: this "needs" keyword is quite a recent feature since gitlab 12.2,
3 months ago: https://docs.gitlab.com/ee/ci/yaml/#needs
5 jobs for master in 25 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #509559
deps-git

00:06:06

passed #509560
deps-x86_64-w64-mingw32

00:15:39

 
  Gimp
passed #509561
gimp-autotools

00:09:29

passed #509562
gimp-meson

00:07:24

passed #509563
gimp-x86_64-w64-mingw32-meson

00:10:17