S1451: FP when pragma is before license header

An issue is raised in the following code snippet (with use of regex copyright [0-9]{4}):

#pragma once
// copyright 2018

void foo();

Because #pragma once is written before the license header.
The same behavior will happen with include guards.

This false positive appeared after the implementation of CPP-2513 that was done to improve the performances of the rule (by not comparing the regex to the whole content of the file, but only to the first lines).

So we need to fix this false positive while keeping the performances of the rule under control (eg: by checking the x first lines of the file)