Fix .policy file translation failure in CentOS CI image (!107)

Build in CentOS 7 CI job fails like this:
    $ make -j $nproc
    ...
    /usr/bin/msgfmt --desktop --template gparted.desktop.in -d ./po -o gparted.desktop
    /usr/bin/msgfmt --xml --template org.gnome.gparted.policy.in -d ./po -o org.gnome.gparted.policy
    /usr/bin/msgfmt: cannot locate ITS rules for org.gnome.gparted.policy.in
    make[2]: *** [org.gnome.gparted.policy] Error 1
    make[2]: *** Waiting for unfinished jobs....
    make[2]: Leaving directory `/builds/mfleetwo/gparted'
    make[1]: *** [all-recursive] Error 1
    make[1]: Leaving directory `/builds/mfleetwo/gparted'
    make: *** [all] Error 2

As with Alpine Linux fixed in the previous commit, CentOS doesn't
install the needed rules files by default.  Add polkit-devel package to
CentOS CI image to fix this.

Closes !107 - Migrate from intltool to gettext translation

.gitlab-ci.yml

@@ -25,7 +25,7 @@ stages:
     - yum update -y
     - yum install -y which gnome-common yelp-tools glib2-devel intltool
                      gcc-c++ libuuid-devel parted-devel gtkmm30-devel make
-                     polkit file
+                     polkit file polkit-devel
     # Extra packages only needed during the test stage.
     # Install EPEL repo first for f2fs-tools and ntfsprogs.
     - yum install -y epel-release