libsoup: add libsoup-gnome support

libsoup can build a libsoup-gnome library. Add support for
libsoup-gnome as it is a dependency of Midori.

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>