base: Update #includes for bitunion.hh.

<iostream> isn't actually used anywhere in bitunion.hh. The templated
hash struct type is defined in <functional> and should be included
explicitly.

Change-Id: I8691ccb2f9e28a01610ae8bb4d9591b07cb7320b
Reviewed-on: https://gem5-review.googlesource.com/7781
Reviewed-by: Matthias Jung <jungma@eit.uni-kl.de>
Maintainer: Gabe Black <gabeblack@google.com>