gdbusprivate: Stop hard-coding path to /var/lib

This will require distributions to ensure they pass
`--localstatedir=/var` correctly to Meson, but they should be doing that
already.

See https://mesonbuild.com/Builtin-options.html#directories for details
about how Meson treats `localstatedir` differently from most other `dir`
variables.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gio/meson.build

@@ -2,6 +2,7 @@ gio_c_args = [
   '-DG_LOG_DOMAIN="GLib-GIO"',
   '-DGIO_COMPILATION',
   '-DGIO_MODULE_DIR="@0@"'.format(glib_giomodulesdir),
+  '-DLOCALSTATEDIR="@0@"'.format(glib_localstatedir),
 ]
 
 gio_c_args += glib_hidden_visibility_args