Do not define emplace_back_aux out-of-line in FBVector (#1077)
Do not define emplace_back_aux out-of-line in FBVector (#1077)

Summary:
- Older versions of GCC required defining private member function
  `emplace_back_aux` out-of-line. That is, declare the member in-class and
  then provide the definition fully out of line.
- These older versions of GCC are not supported anymore, so just define
  it inline as you would any other member function.
Pull Request resolved: https://github.com/facebook/folly/pull/1077

Reviewed By: meyering

Differential Revision: D14591465

Pulled By: yfeldblum

fbshipit-source-id: 0ebfe55cec4e230c36c98ad595ec8dcfc98a211e

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly