package/matchbox: forcibly disable gconf

Buildroot does not have a gconf package, so explicitly disable gconf
support (to avoid it ends up picking the host one).

Reported-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>