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