bitbake.conf: change GNOME_MIRROR to new one

URI has been permanently moved and returns HTTP 301.

Signed-off-by: Oleksandr Kravchuk <open.source@oleksandr-kravchuk.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit a3a85d54)
Signed-off-by: Steve Sakoman <steve@sakoman.com>