Fix THD->c10 dependency to gflags.h (#20319)
Summary:
Fixed #20250
Not sure if there's any specific design reason to `add_dependecy()` and manually add a few include dir, instead of linking the target.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20319
Differential Revision: D15294584
Pulled By: ezyang
fbshipit-source-id: 97f813a6b1829dad49958e0f880b33eb95747607
