gtk: Don't vte_terminal_set_encoding() on new VTE versions

gtk: Don't vte_terminal_set_encoding() on new VTE versions

The function vte_terminal_set_encoding() is deprecated since VTE 0.54,
so stop calling it from that version on. This fixes a build error
because of our use of warning flags [-Werror=deprecated-declarations].

Fixes: https://bugs.launchpad.net/bugs/1794939
Reported-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20181011153039.2324-1-kwolf@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 6415994ffcc6d22b3f5add67f63fe77e4b9711f4)