Enable PolyKinds for Control.Category.

This change will allow us to make various new things instances of
Category, such as promoted tuples.

Original discussion from libraries@haskell.org:

  http://www.haskell.org/pipermail/libraries/2012-August/018268.html

Fell off the radar. No complaints in the 2 week period since being
brought up again:

  http://www.haskell.org/pipermail/libraries/2013-May/019845.html

Patch from Edward Kmett (essentially.)

Authored-by: Edward Kmett <ekmett@gmail.com>
Signed-off-by: Austin Seipp <aseipp@pobox.com>