Set axis hidden flag on all Parametric Axes

Currently all parametric axes are 'public', so @behdad 's new avar2 partial-instancer ignores them; so please add the flags. Built TTF's TTX should change from e.g. user/public axis then parametric/flagged axis:

    <!-- Grade -->
    <Axis>
      <AxisTag>GRAD</AxisTag>
      <Flags>0x0000</Flags>
      <MinValue>-250.0</MinValue>
      <DefaultValue>0.0</DefaultValue>
      <MaxValue>150.0</MaxValue>
      <AxisNameID>260</AxisNameID>
    </Axis>

    <!-- Parametric Spacing -->
    <Axis>
      <AxisTag>XTSP</AxisTag>
      <Flags>0x0000</Flags>
      <MinValue>-100.0</MinValue>
      <DefaultValue>0.0</DefaultValue>
      <MaxValue>100.0</MaxValue>
      <AxisNameID>261</AxisNameID>
    </Axis>

to

    <!-- Grade -->
    <Axis>
      <AxisTag>GRAD</AxisTag>
      <Flags>0x0000</Flags>
      <MinValue>-250.0</MinValue>
      <DefaultValue>0.0</DefaultValue>
      <MaxValue>150.0</MaxValue>
      <AxisNameID>260</AxisNameID>
    </Axis>

    <!-- Parametric Spacing -->
    <Axis>
      <AxisTag>XTSP</AxisTag>
      <Flags>0x0001</Flags>
      <MinValue>-100.0</MinValue>
      <DefaultValue>0.0</DefaultValue>
      <MaxValue>100.0</MaxValue>
      <AxisNameID>261</AxisNameID>
    </Axis>