Mark protobuf-compiler-grpc Multi-Arch: foreign

Andreas Henrikssonrequested to merge
ah/multiarch-foreign-compiler into master

It contains plugins for protobuf-compiler, which is also already Multi-Arch: foreign. Mixing different architectures of protoc (protobuf-compiler) and grpc_cpp_plugin (protobuf-compiler-grpc) seems to work....

Closes: #976651 Thanks: Helmut for valuable insight and review.

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