folly/io/async/ScopedEventBaseThread.h: mark overriding functions with "override"
folly/io/async/ScopedEventBaseThread.h: mark overriding functions with "override"

Summary:
This avoids the following errors:

  folly/io/async/ScopedEventBaseThread.h:49:3: error: '~ScopedEventBaseThread' overrides a destructor but is not marked 'override' [-Werror,-Wsuggest-destructor-override]

Reviewed By: ryanthomasjohnson

Differential Revision: D33397489

fbshipit-source-id: 29210c0de0b850bdda5d2c27a5be8971fc17b40e

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