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>