[gtk] ci/docker: add lcov so we can create coverage reports

[Christoph Reiter <creiter src gnome org>]

commit 58cdd5139e7ba93fd9a7d52da72872027b0ddfb7
Author: Christoph Reiter <creiter src gnome org>
Date:   Tue Sep 18 09:55:50 2018 +0200

    ci/docker: add lcov so we can create coverage reports
    
    This was requested on IRC; the docker image in the registry is updated already.

 .gitlab-ci/Dockerfile | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci/Dockerfile b/.gitlab-ci/Dockerfile
index f5e6f22513..52dcaa1985 100644
--- a/.gitlab-ci/Dockerfile
+++ b/.gitlab-ci/Dockerfile
@@ -33,6 +33,7 @@ RUN dnf -y install \
     iso-codes \
     itstool \
     json-glib-devel \
+    lcov \
     libattr-devel \
     libepoxy-devel \
     libffi-devel \