meson: fix systemd-pot target when polkit-devel is not installed

To successfully extract strings from our .policy files, gettext needs
polkit.{its,loc} files provided by policykit-devel. When that package is not
installed, systemd-pot would fail to extract strings:

[0/1] Running external command systemd-pot.
xgettext: warning: file 'src/core/org.freedesktop.systemd1.policy.in.in' extension 'policy' is unknown; will try C
xgettext: warning: file 'src/hostname/org.freedesktop.hostname1.policy.in' extension 'policy' is unknown; will try C
...

We now don't need the .its and .loc files for normal building, but they are
still useful when generating the .pot file, because that way we avoid the
dependency on sufficiently new polkit. We just need to tell i18n to pass their
location to xgettext.

(cherry picked from commit b0faead2)