Fix running protoc

[kolyshkin]

A recent commit (PR #20) caused a regression:

protoc --decode google.protobuf.FileDescriptorSet /usr/local/include/google/protobuf/descriptor.proto
/usr/local/include/google/protobuf/descriptor.proto: File does not reside within any path specified using --proto_path (or -I). You must specify a --proto_path which encompasses this file. Note that the proto_path must be an exact prefix of the .proto file names -- protoc is too dumb to figure out when two paths (e.g. absolute and relative) are equivalent (it's harder than you think).

Although I can't seem to reproduce it locally, it is clear that
protoc do require --proto_path in this case. Add it back.

Fixes: 95af7b7

[kolyshkin]

@stevvooe PTAL

[kolyshkin]

Maybe it's not needed for protoc 3.6.1, but it def fails with 3.5.0

[kolyshkin]

The failure can be seen in moby/swarmkit#2750 (https://circleci.com/gh/docker/swarmkit/9619?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link)

[kolyshkin]

Maybe it's not needed for protoc 3.6.1, but it def fails with 3.5.0

Indeed it works fine with 3.6.1:
https://circleci.com/gh/docker/swarmkit/9641?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
(see the "Compile/lint/vet/protobuf validation" step)

[kolyshkin]

This looks ugly, and is not needed for protoc 3.6.1, so feel free to close

[stevvooe]

@kolyshkin Let me know if that changes!