Fix compilation on GHC 7.8.1 RC2

Before this patch, GHC 7.8.0.20140226 failed with:

    src/Control/Lens/Internal/Exception.hs:192:61:
    Ambiguous occurrence ‘Typeable1’
    It could refer to either ‘Control.Lens.Internal.Exception.Typeable1’,
                             defined at src/Control/Lens/Internal/Exception.hs:44:1
                          or ‘Data.Typeable.Typeable1’,
                             imported from ‘Data.Typeable’ at src/Control/Lens/Internal/Exception.hs:41:1-