Logcat Text: drop use of GDateTime

It's only available from GLib 2.26 while we only require 2.16

Change-Id: I3c2e6748aa93b9af7158f5051433baff6a7c0324
Reviewed-on: https://code.wireshark.org/review/3480
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>