syncqt/qmake not generating correct install rules for inject_headers

When adding inject_headers in sync.profile, qmake won't generate correct install rules for these injected headers. 

This can be seen in the attempt to generate files through qlalr in qtdeclarative at build time.

https://codereview.qt-project.org/#/c/219812/ won't currently integrate, as shadow builds generate wrong install rules for the generated private headers trying to find them in the source dir, not the build dir.

http://10.212.3.36:8080/coin/integration/qt/qtdeclarative/tasks/web_qt_qtdeclarative_1521194143873 shows such a failed integration.