ci: Build Broadway on MacOS

We've seen a build failure if Broadway is built without
the X11 backend. Lets recreate that configuration in ci
so it doesn't break again.

.gitlab-ci.yml

@@ -170,6 +170,8 @@ macos:
     - export MESON_FORCE_BACKTRACE=1
   script:
     - meson -Dx11-backend=false
+            -Dbroadway-backend=true
+            -Dmacos-backend=true
             -Dintrospection=disabled
             -Dcpp_std=c++11
             -Dpixman:tests=disabled