spice-common: don't install spice-protocol
spice-common: don't install spice-protocol

Revert to usage of DIST_SUBDIRS to avoid make install under
spice-protocol. This is only possible since git.mk was fixed.

fgouget/spice-common,fgouget/spice-common,freedesktop-unofficial-mirror/spice__spice-common,Fantu/spice-common,elmarco/spice-common,SPICE/spice-common,elmarco/spice-common,fgouget/spice-common,SPICE/spice-common,freedesktop-unofficial-mirror/spice__spice-common,freedesktop-unofficial-mirror/spice__spice-common,Fantu/spice-common,elmarco/spice-common,Fantu/spice-common,freedesktop-unofficial-mirror/spice__spice-common,elmarco/spice-common,Fantu/spice-common,fgouget/spice-common,SPICE/spice-common