Extcap: restore compatibility with GLib 2.16.0

While we are at it, fix what seem to be a memory leak in extcaps_init_initerfaces()

Change-Id: I1bb9a1b44d16f986eedd192b15cce84c5881a917
Reviewed-on: https://code.wireshark.org/review/3820
Reviewed-by: Roland Knall <rknall@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>