Makefile: Add check for out of date protocol buffer code
Makefile: Add check for out of date protocol buffer code

'make check' will now fail if the files produced by re-running protoc
differ from those which are checked into the repository.

Signed-off-by: Euan Harris <c3b6e83069c8e9e3af49a38fec6026be89559638@docker.com>

Ramzec/docker,vdemeester/docker,bearstech/moby,jameszhan/docker,yosifkit/docker,mahak/docker,Mokolea/docker,bearstech/moby,jimjag/docker,jthelin/docker,mahak/docker,jameszhan/docker,jimjag/docker,moby/moby,mahak/docker,gnailuy/docker,trebonian/docker,paralin/docker,vdemeester/docker,flavio/docker,djs55/docker,nawawi/docker,nalind/docker,cpuguy83/docker,paralin/docker,cpuguy83/docker,cpuguy83/docker,Ramzec/docker,vdemeester/docker,gnailuy/docker,moby/moby,moby/moby,jameszhan/docker,jimjag/docker,paralin/docker,AkihiroSuda/docker,gnailuy/docker,moul/docker,thaJeztah/docker,Mokolea/docker,trebonian/docker,yosifkit/docker,cpuguy83/docker,trebonian/docker,Ramzec/docker,moul/docker,flavio/docker,trebonian/docker,nawawi/docker,konstruktoid/docker-upstream,AkihiroSuda/docker,moul/docker,flavio/docker,moul/docker,Mokolea/docker,jthelin/docker,jimjag/docker,nalind/docker,nalind/docker,nalind/docker,yosifkit/docker,mahak/docker,gnailuy/docker,nawawi/docker,thaJeztah/docker,konstruktoid/docker-upstream,docker/docker,flavio/docker,thaJeztah/docker,thaJeztah/docker,Mokolea/docker,AkihiroSuda/docker,konstruktoid/docker-upstream,bearstech/moby,AkihiroSuda/docker,konstruktoid/docker-upstream,docker/docker,bearstech/moby,djs55/docker,djs55/docker,docker/docker,nawawi/docker,moby/moby,Ramzec/docker,yosifkit/docker,paralin/docker