build-sys: Use SPICE_WARNING() to issue the DBus warning
build-sys: Use SPICE_WARNING() to issue the DBus warning

Signed-off-by: Francois Gouget <6b915da68578f6eb004fe99432b967565fc9be8f@codeweavers.com>

SPICE/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk