grpc C++ plugin generates additional files, namely .grpc.pb.cc and
.grpc.pb.h. Adding these files to the outs of the _proto_gen rule, so
dependents don't complain about undeclared inclusions. Also, compiling
the .grpc.pb.cc requires additional header files from the grpc library,
so added //external:grpc_lib to the deps of the
cc_library. Clients are expected to declare that in their bazel
WORKSPACE, pointing it to @grpc//:grpc++{_unsecure}.