main/postmarketos-base-ui-gnome: fix polkit conflict (MR 5885)

Add polkit-elogind to _pmb_recommends, to help apk resolve the polkit
depends again. Otherwise it fails with a conflict since recent changes
in apk's dependency resolving code.

Fixes: issue 3349
[ci:skip-vercheck]: pmb_recommends change doesn't need rebuild
[ci:skip-build]: already built successfully in CI

(cherry picked from commit 954fee66)