polkit: don't build with gobject-introspection when cross-compiling
polkit: don't build with gobject-introspection when cross-compiling

gobject-introspection doesn't currently cross-compile (see
https://github.com/NixOS/nixpkgs/pull/88222), but polkit is somewhat
essential for many system components.

By disabling gobject-introspection when cross-compiling, we get it to
build.

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