package/spice: depend on libglib2

The package selects BR2_PACKAGE_LIBGLIB2 but did not depend on it.

The buildsystem treats libglib2 as a hard-dependency:
https://cgit.freedesktop.org/spice/spice/tree/configure.ac?h=0.12#n117



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>