Summary:
... so that we have freedom to upgrade it (see https://github.com/facebook/rocksdb/issues/6808).

As a side benefit, gtest will no longer be linked into main library in
buck build.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6858

Test Plan: fb internal build & link

Reviewed By: riversand963

Differential Revision: D21652061

Pulled By: pdillinger

fbshipit-source-id: 6018104af944debde576b5beda6c134e737acedb