The newly-introduced functions, g_content_type_get_symbolic_icon() and
g_content_type_get_generic_icon_name() don't seem to be for Windows, at
least for now.  So filter them out from gio.symbols on Windows.

Also, glocalfileinfo.c calls g_content_type_get_symbolic_icon() in
get_icon(), so only build that code when on Unix, for the time being.

https://bugzilla.gnome.org/show_bug.cgi?id=684278