package/polkit: fix install of polkit.its

package/polkit: fix install of polkit.its

Install polkit.its in $(HOST_DIR)/share/gettext/its as suggested by
https://gitlab.gnome.org/GNOME/gvfs/-/issues/508 to avoid the following
build failure with gvfs because polkit installs polkit.its in
$(STAGING_DIR)/usr/share/gettext:

FAILED: daemon/org.gtk.vfs.file-operations.policy
/tmp/instance-3/output-1/host/bin/meson --internal msgfmthelper daemon/org.gtk.vfs.file-operations.policy.in daemon/org.gtk.vfs.file-operations.policy xml /tmp/instance-3/output-1/build/gvfs-1.44.1/po
msgfmt: cannot locate ITS rules for daemon/org.gtk.vfs.file-operations.policy.in

Fixes:
 - http://autobuild.buildroot.org/results/ba9a8b31e6e275d4db1905241a9153a0513eb335

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>