Fix build with Qt 5.10

Summary: QByteArray does not include QString anymore, but QString is used in k_pty.h

Test Plan: Did not build before, now does.

Reviewers: Frameworks, lbeltrame

Reviewed By: lbeltrame

Subscribers: lbeltrame, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8070