exclude .pluginspec from 'make clean'

'clean' per qmake's definition means "all intermediate artifacts are
gone". the target is supposed to be still runnable/installable.

Task-number: QTCREATORBUG-10323
Change-Id: Ic0d6482c7534367753661e80ccd5f55bd0446545
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>