package/polkit: switch to meson-package

Switch to meson-package to fix the following build failure raised since
bump of gobject-introspection to version 1.68.0 in commit
abc110e3:

Could not find GIR file 'Gio-2.0.gir'; check XDG_DATA_DIRS or use --includedir
error parsing file Polkit-1.0.gir: Failed to parse included gir Gio-2.0
If the above error message is about missing .so libraries, then setting up GIR_EXTRA_LIBS_PATH in the .mk file should help.
Typically like this: PKG_MAKE_ENV += GIR_EXTRA_LIBS_PATH="$(@D)/.libs"

Fixes:
 - http://autobuild.buildroot.org/results/26bcb21900b403db690d6005dbef0a76ff494d6c



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>