[bqschemaupdater] Include gRPC in proto import path
[bqschemaupdater] Include gRPC in proto import path

Like cproto, bqschemaupdater also compiles protos.
Add support for gRPC protos.

Change-Id: I5d974710fdc39fa88ef594a12eb4ae8007f57b05
Reviewed-on: https://chromium-review.googlesource.com/1113815
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go