heap_vector_types: unbreak getKey() with rvalue
heap_vector_types: unbreak getKey() with rvalue

Summary: D40414865 (https://github.com/facebook/folly/commit/fe8cf3dca13cf7d38b86a684ebab47dd097dbf06) broke this.

Reviewed By: Gownta

Differential Revision: D40496222

fbshipit-source-id: 1184a42ca90ba83b7076cc36e394f8337bcf36d5

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