Fix GHC build with text >=1.2.5

[amesgen]

Without this patch, we have

 $ cabal build --constraint='text >=1.2.5'
src/Miso/FFI.hs:95:35: error:
    Ambiguous occurrence ‘elem’
    It could refer to
       either ‘Prelude.elem’,
              imported from ‘Prelude’ at src/Miso/FFI.hs:14:8-15
              (and originally defined in ‘Data.Foldable’)
           or ‘Miso.String.elem’,
              imported from ‘Miso.String’ at src/Miso/FFI.hs:67:1-28
              (and originally defined in ‘Data.Text’)
   |
95 |   classSet <- ((JSS.pack "class") `elem`) <$> listProps obj
   |                                   ^^^^^^

where elem was added to Data.Text in 1.2.5.0.

[dmjio]

@amesgen thanks