rpc: update with newer protoc-gen-go
rpc: update with newer protoc-gen-go

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	# (bring in latest protoc-gen-go)
	go get github.com/golang/protobuf/protoc-gen-go
	# (regen for github.com/golang/protobuf@v1.4)
	make -B rpc/rpc.pb.go

This prepares the code to be switched to google.golang.org/protobuf/proto

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

checkpoint-restore/go-criu,checkpoint-restore/go-criu,checkpoint-restore/go-criu