[Go]: Fixup paths to protocol buffers.

Without this: go generate github.com/tensorflow/tensorflow/tensorflow/go/op
would fail with:
../genop/internal/api_def_map.go:34:2: cannot find package "github.com/tensorflow/tensorflow/tensorflow/go/genop/internal/proto/tensorflow/core/framework_go_proto" in any of:
        /usr/local/go/src/github.com/tensorflow/tensorflow/tensorflow/go/genop/internal/proto/tensorflow/core/framework_go_proto (from $GOROOT)
        /go/src/github.com/tensorflow/tensorflow/tensorflow/go/genop/internal/proto/tensorflow/core/framework_go_proto (from $GOPATH)

This breakage was probably introduced by https://github.com/tensorflow/tensorflow/pull/17262