don't install .lib files under windows

we don't support building 3rd party plugins against an installed qt
creator anyway (we don't install any headers), so it makes no sense to
install import libraries (especially into the wrong directory).

Task-number: QTCREATORBUG-8395
Change-Id: Ie6703598324d9dd39aa82d1eb0f2fecd224e4390
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>