[gnome-flashback] ci: add libgdm-dev

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 94cd83e3c4b42bd800ff06331f4884662060fb36
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sun Feb 9 14:55:24 2020 +0200

    ci: add libgdm-dev

 .gitlab-ci.yml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 2f9e148..4e97944 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -21,6 +21,7 @@ build-ubuntu:
                       libcanberra-gtk3-dev
                       libdbus-glib-1-dev
                       libgdk-pixbuf2.0-dev
+                      libgdm-dev
                       libglib2.0-dev
                       libgnome-bluetooth-dev
                       libgnome-desktop-3-dev