Teach the style checker about common headers living in gem5/. These should be included after any global library headers (e.g., C headers or STL headers), but before the normal gem5 headers. Change-Id: I322f841420e361c16314be8fa4cbd1e86d2bfa9f Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com> Reviewed-on: https://gem5-review.googlesource.com/4300 Reviewed-by: Jason Lowe-Power <jason@lowepower.com>