meson: Depend on GLib >= 2.68
fedefd99 "Remove GWeatherTimezone" says:
GWeatherTimezone can be replaced by GTimeZone, which has been available since GLib 2.26.
It is true that GTimeZone itself was added in 2.26. However, this library uses g_time_zone_new_identifier(), which was added in GLib 2.68 as a replacement for the deprecated g_time_zone_new().