client/windows: use SPICE_PROTOCOL_DIR in project include dirs
client/windows: use SPICE_PROTOCOL_DIR in project include dirs

instead of ..\..\..\spice-protocol. Relative path to another git tree is a bit
ugly, since it requires spice-protocol to be in a specific location.

SPICE_PROTOCOL_DIR should also be used in windows qxl and vdagent instead of
SPICE_COMMON_DIR, which is an old and confusing name, due to the common
directory in spice git repo.

SPICE/spice,dezelin/spice,elmarco/spice,dezelin/spice,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,Fantu/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,ya-mouse/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,alon/spice-server,alon/spice-server,elmarco/spice,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,alon/spice-server,fgouget/spice,elmarco/spice,fgouget/spice,dezelin/spice,Fantu/spice,mathslinux/spice,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,SPICE/spice,mathslinux/spice,SPICE/spice,Fantu/spice,dezelin/spice,ya-mouse/spice,alon/spice-server,mathslinux/spice,ya-mouse/spice,dezelin/spice