thrift/lib/cpp2/transport/core/ThriftRequest.h: fix llvm12-exposed build error
thrift/lib/cpp2/transport/core/ThriftRequest.h: fix llvm12-exposed build error

Summary:
Add the override attribute to avoid this new warning/error:

  thrift/lib/cpp2/transport/core/ThriftRequest.h:521:3: error: '~ThriftRequest' overrides a destructor but is not marked 'override' [-Werror,-Wsuggest-destructor-override]

Differential Revision: D32056927

fbshipit-source-id: 1cabc21e214903c291c1ef0732d80e2b4412fe19

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift