Add a check for the "We mean it" comment of private headers to the gerrit sanity bot

The comment is often forgotten when adding new private headers, causing fex

• https://codereview.qt-project.org/#/c/177936/
• https://codereview.qt-project.org/#/c/150243/

A check could be added to the gerrit sanity bot, fex based on the *_p.h pattern.