Export meta_get_replace_current_wm()

This is necessary for gnome-shell to know whether it should try to
replace the org.gnome.Shell DBus name.

Note: We can't just #include <meta/util.h> because it #defines '_'
at least which obviously conflicts with core gettext.  We should
un-export util.h or fix it.

https://bugzilla.gnome.org/show_bug.cgi?id=645590