Generalise type of recently added System.Exit.die

This is a follow-up to 77ea2eb0ab36d1a (re #9016) which added `die` with a
return type of `IO ()` even though all other functions in System.Exit have
the more general return type `IO a`. It is assumed this was an oversight in
the original proposal.

Acked-by: Edward Kmett <ekmett@gmail.com>
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>