QStringTokenizer: Add a missing include

This header uses std::forward_iterator_tag, which requires including
the <iterator> header.

This fixes building with libc++ with
_LIBCPP_REMOVE_TRANSITIVE_INCLUDES enabled.

Pick-to: 6.7 6.5 6.2
Change-Id: Id2ce97e158c87dab1efe30e54a93f0bc9351de5a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/text/qstringtokenizer.h

@@ -5,6 +5,7 @@
 
 #include <QtCore/qnamespace.h>
 #include <QtCore/qcontainerfwd.h>
+#include <iterator>
 
 QT_BEGIN_NAMESPACE