Fix OSS build
Fix OSS build

Summary:
As per https://github.com/facebook/fbthrift#build we supposed to run cmake from inside `build` folder and not `thrift`.

Verified this fix on my Ubuntu18.04 VirtualBox.

Reviewed By: andreazevedo

Differential Revision: D14194346

fbshipit-source-id: 9cb9dfc2cc8e569d64f299436684c866a60eee5b

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter