gtk: Don't use deprecated vte_terminal_get_adjustment

Guard this with a VTE version check, since I'm not sure if this is backwards
compatible.

ui/gtk.c: In function ‘gd_vc_init’:
ui/gtk.c:1176:5: error: ‘vte_terminal_get_adjustment’ is deprecated (declared at /usr/include/vte-2.90/vte/vtedeprecated.h:101) [-Werror=deprecated-declarations]

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
