Use insert in place of initializer list to initialize flat_set.
Use insert in place of initializer list to initialize flat_set.

Summary: In older versions of the boost library, flat_set does not have a constructor which accepts an initializer list. To accomodate these older versions, flat_set is initialized by expliciting calls to insert().

Reviewed By: capveg

Differential Revision: D5539093

fbshipit-source-id: c5a77e34f4454c95c13d2c11fa49bc788056d322

peterlei/fboss,peterlei/fboss,peterlei/fboss,peterlei/fboss