wp: make wp_get_xdg_state_dir() private

It's not used elsewhere, so there is no good reason yet to expose it.
Besides, I expect that GLib will likely add a method in the future
to return the XDG_STATE_HOME (which is relatively new in the xdg spec)
7 jobs for config-funcs in 1 minute and 33 seconds (queued for 8 seconds)
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