[CRIU] [PATCH] make: fix build of protobuf/ directory

[Filipe Brandenburger]

BTW, I don't seem to have problems with the dependency of other
protobufs on opts.proto, for example:

$ make clean
... (removing sources)
$ git clean -fdx
$ make protobuf/core-x86.pb-c.c
...
  PB DDEP  protobuf/core-x86.proto.d
  PB DEP   protobuf/core-x86.proto.c.d
...
make[1]: Leaving directory `/usr/local/google/home/filbranden/devel/criu'
make[1]: Entering directory `/usr/local/google/home/filbranden/devel/criu'
  PB DDEP  protobuf/opts.proto.d
  PB DEP   protobuf/opts.proto.c.d
  PBCC     protobuf/opts.pb-c.c
  PBCC     protobuf/core-x86.pb-c.c
make[1]: Leaving directory `/usr/local/google/home/filbranden/devel/criu'