fix: add missing rdf type to custom parameter units

[SpotlightKid]

Without these, checking the plugins with lv2lint, gives errors like these:

<http://moddevices.com/plugins/mda/Degrade>
  {1 : quant}
    [FAIL]  Port Units
              units_unit not a URI or object
              seeAlso: <http://lv2plug.in/ns/extensions/units#unit>
  {2 : rate}
    [FAIL]  Port Units
              units_unit not a URI or object
              seeAlso: <http://lv2plug.in/ns/extensions/units#unit>
  {5 : non_lin}
    [FAIL]  Port Units
              units_unit not a URI or object
              seeAlso: <http://lv2plug.in/ns/extensions/units#unit>

See also https://lv2plug.in/ns/extensions/units (second example).