Allow `Makefile` files to contain tabs.

Summary: Add an entry to the `.editorconfig` file to allow `Makefile` files to contains tabs over spaces. Without this entry, my editor will automatically convert the tabs to spaces (which `make` does not like).

Test Plan: Opened `./support/xhpast/Makefile` and saved it. Verified that spaces weren't converted to tabs.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9436