Use QT_INSTALL_HEADERS instead of QT_INSTALL_PREFIX/include

It is more accurate this way as there might be scenarios, like Red Hat, where
the headers are not installed into $$QT_INSTALL_PREFIX/include.

Task-number: QTBUG-31966
Change-Id: I4be31a4b218aed008fecba81e8d2b91616cea370
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>