Do not warn for header-hygiene.

QtDeclarative is consciously leaking the namespace QtQml.
Therefore clang would warn about header hygiene.
Unfortunately this cannot be fixed in QtDeclarative,
unless we accept not to be able to use QtQuick1 and QtQuick2
in the same application.

Change-Id: I5ba78eb20f98e2e04d9c190794c83710b7f196e4
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>