gtimezone: Tidy up UTC timezone creation
Move the separate function inline using g_once_init_{enter,leave}(), rather than g_once().
This marginally improves performance, taking 0.39s to create 10000000 UTC timezones, rather than 0.43s previously. (Who cares?)
Follow-up to !1105 (merged).
Signed-off-by: Philip Withnall withnall@endlessm.com