Unconditionally enable -DQT_STRICT_ITERATORS, not just in debug mode

Summary:
It turns out QT_STRICT_ITERATORS does not introduce any
overhead (they are inlined so everything gets optimized), confirmed by
Dan comparing the produced assembly in Compiler Explorer.

Reviewers: dvratil, mlaurent, aacid

Reviewed By: aacid

Subscribers: aacid, kde-buildsystem, kde-frameworks-devel

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D20349