cabal: turn off C++ -Wnullability-completeness warning (#488)

Summary:
This stops the C++ compiler from spitting out a little over 18,000 lines of C++ warnings about pointer nullability, which makes the GitHub logs actually unusable.

Pull Request resolved: #488

Reviewed By: josefs

Differential Revision: D68622988

Pulled By: simonmar

fbshipit-source-id: 95f3d657fb78c149467ffdef72272e1f29f22d6a

glean.cabal.in

@@ -52,7 +52,7 @@ common fb-haskell
      ghc-options: -O2
 
 common fb-cpp
-  cxx-options: -std=c++17
+  cxx-options: -std=c++17 -Wno-nullability-completeness
   if !flag(clang)
      cxx-options: -fcoroutines
   if arch(x86_64)