Initialize policydb.process_class eariler.

Initialize policydb.process_class once all symtabs read from policy image,
so that it could be used to setup the role_trans.tclass field when a lower
version policy.X is loaded.

Signed-off-by: Harry Ciao <qingtao.cao@windriver.com>
Signed-off-by: Eric Paris <eparis@redhat.com>