gtk: fix vte version check

vte_terminal_set_encoding takes 3 args since 0.38.0.
This fixes commit fba958c6 ("gtk: implement set_echo")

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Message-id: 20160608214352.32669-1-olaf@aepfle.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
