Make QServicePackagePrivate::clean non-virtual
Make QServicePackagePrivate::clean non-virtual

This class method does not need to be virtual, since nothing is
deriving from it. If something ever does, then the destructor should
be virtual too, to avoid this warning:

qservicepackage.cpp:49:10:   required from here
qshareddata.h:163:72: warning: deleting object of polymorphic class type ‘QServicePackagePrivate’ which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]

Change-Id: I0e5ccc6edcb6a74e801edfbe73320cf5a0ece171
Reviewed-by: Aaron McCarthy <fd47055c798be0f4eb798b431595a9333a5352a4@gmail.com>

qtproject/qtsystems,qtproject/qtsystems