[pan] fix make distcheck for gtk2

[Dominique Dumont <ddumont src gnome org>]

commit 9bb4d1819fa73e312583630cc5a32b93d6365ede
Author: Dominique Dumont <dod debian org>
Date:   Mon Jan 10 19:48:31 2022 +0100

    fix make distcheck for gtk2
    
    Make distcheck runs configure without any option. Since Gtk3 is now
    used by default, this steps also build pan with gtk3.
    
    There may be a solution where make distcheck also uses Gtk2, but I
    won't bother since Gtk2 is deprecated.

 .gitlab-ci.yml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 32d01e2..a06f30b 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -19,6 +19,7 @@ build-ubuntu-gtk2:
         gettext
         gcc
         libgtk2.0-dev
+        libgtk-3-dev # required by make distcheck
         libgmime-3.0-dev
         libnotify-dev
         gnome-keyring