Improve the check for header guard

Summary:
cpplint.py only recognize `#ifdef HEADER_GUARD` as header guard.
This patch enables the check for `#pragma once`.

Test Plan: New arc lint exclude the false alarm for `#pragma once`.

Reviewers: dhruba, sdong, igor, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16227