Add polkitd to CI build

For gettext migration of project containing a policykit file.

Dockerfile

@@ -10,6 +10,7 @@ RUN apt-get update \
   libxss-dev libindicator3-dev libxmu-dev libburn-dev libisofs-dev  libpulse-dev libkeybinder-3.0-dev libmpd-dev valac libvala-0.56-dev gobject-introspection libgirepository1.0-dev librsvg2-dev libtagc0-dev libdbusmenu-gtk3-dev libgtop2-dev libnotify-dev libxklavier-dev libexif-dev libgudev-1.0-dev libupower-glib-dev libclutter-1.0-dev libsensors4-dev libical-dev \
   libwayland-bin libwayland-dev libgtk-layer-shell-dev \
   libcurl4-openssl-dev libffmpegthumbnailer-dev libgsf-1-dev libpoppler-glib-dev libopenrawgnome-dev libgepub-0.6-dev \
+  polkitd \
   && rm -rf /var/lib/apt/lists/*
 
 # Build and install the latest tag for all Xfce core libraries