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 <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>
Message-id: 20181011153039[email protected]
Signed-off-by: Gerd Hoffmann <[email protected]>