grpc: depend on re2

[SMillerDev]

• Have you followed the guidelines for contributing?
  -[x] Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---

Should fix #61151

[bayandin]

It looks like bloaty and dnsdist require a revision bump:

Error: 4 failed steps!
brew linkage --test bloaty
brew test --retry --verbose bloaty
brew linkage --test dnsdist
brew test --retry --verbose dnsdist

[BrewTestBot]

🤖 A scheduled task has triggered a merge.

[fxcoudert]

I think this is causing apache-arrow to fail building from source now with:

2020-10-10T19:57:41.2826500Z CMake Error at cmake_modules/Findre2.cmake:65 (add_library):
2020-10-10T19:57:41.2827150Z   add_library cannot create imported target "RE2::re2" because another target
2020-10-10T19:57:41.2827850Z   with the same name already exists.
2020-10-10T19:57:41.2828250Z Call Stack (most recent call first):
2020-10-10T19:57:41.2829030Z   /usr/local/lib/cmake/grpc/gRPCConfig.cmake:21 (find_package)
2020-10-10T19:57:41.2829760Z   cmake_modules/ThirdpartyToolchain.cmake:2453 (find_package)
2020-10-10T19:57:41.2830370Z   CMakeLists.txt:484 (include)

which is blocking #62560