misc: fix build failure in cxx_config example

misc: fix build failure in cxx_config example

Fix a missing header in the cxx_config example which is used as a
simple example of using libgem5.so without python.

Change-Id: I758bfe42ba735ce0c7eaedd49b94a130e3bd21e3
Reviewed-on: https://gem5-review.googlesource.com/3000
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Matthias Jung <jungma@eit.uni-kl.de>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>