(maint) Define Util::Resolution#resolution_type

The resolution_type method is needed when reopening a fact definition
with the Facter 2.0 API:

Facter.define_fact(:myfact) do
  define_resolution(:myres) do
    # [...]

The aggregate resolution type implemented this but Util::Resolution did
not, which could cause errors using the new syntax. This corrects the
omission.