ci: Add CI

This just builds the "what-did-you-plugin" test application, but it's
good enough to catch the worst failures.

.gitlab-ci.yml

+stages:
+- test
+
+build-fedora:
+  image: fedora:latest
+  stage: test
+  before_script:
+    - dnf install -y redhat-rpm-config gcc make pulseaudio-libs-devel alsa-lib-devel gtk3-devel
+  script:
+    - make -f Makefile.tests
+