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 <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-by: Curtis Dunham <14490907fdc9c150c92157b82121c81003bbdc71@arm.com>
Reviewed-by: Matthias Jung <2d08bb393b588a7f78bde56e4f99e16f4aa802a1@eit.uni-kl.de>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>

Weil0ng/gem5,TUD-OS/gem5-dtu,sobercoder/gem5,gem5/gem5,gedare/gem5,gedare/gem5,rallylee/gem5,gem5/gem5,sobercoder/gem5,Weil0ng/gem5,sobercoder/gem5,powerjg/gem5-ci-test,Weil0ng/gem5,TUD-OS/gem5-dtu,Weil0ng/gem5,gedare/gem5,sobercoder/gem5,rallylee/gem5,TUD-OS/gem5-dtu,powerjg/gem5-ci-test,gedare/gem5,TUD-OS/gem5-dtu,rallylee/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,rallylee/gem5,gem5/gem5,gedare/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,Weil0ng/gem5,sobercoder/gem5,sobercoder/gem5,gem5/gem5,gedare/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gedare/gem5,rallylee/gem5,Weil0ng/gem5,Weil0ng/gem5,gem5/gem5,powerjg/gem5-ci-test,rallylee/gem5,sobercoder/gem5,powerjg/gem5-ci-test,rallylee/gem5