Fix moxygen OSS build

Summary:
Fixes a breakage in the open source build of moxygen, see:

```
/Users/jcenzano/MetaOpenSource/moxygen/_build/deps/fizz/fizz/test/BogoShim.cpp:396:3: error: reference to 'gflags' is ambiguous
  396 |   gflags::ParseCommandLineFlags(&argc, &argv, true);
      |   ^
/Users/jcenzano/homebrew/include/gflags/gflags_gflags.h:41:11: note: candidate found by name lookup is 'gflags'
   41 | namespace gflags {
      |           ^
/Users/jcenzano/MetaOpenSource/moxygen/_build/deps/include/folly/portability/GFlags.h:124:11: note: candidate found by name lookup is 'folly::gflags'
  124 | namespace gflags = ::GFLAGS_NAMESPACE;
      |           ^
```

Reviewed By: afrind

Differential Revision: D66572281

fbshipit-source-id: fe02d944e07cbf4daf7b93bf4dc410226e99d10a

fizz/test/BogoShim.cpp

@@ -393,7 +393,7 @@ int main(int argc, char** argv) {
     }
   }
 
-  gflags::ParseCommandLineFlags(&argc, &argv, true);
+  folly::gflags::ParseCommandLineFlags(&argc, &argv, true);
   google::InitGoogleLogging(argv[0]);
   CryptoUtils::init();