scripts/setup.py: Fix prerequisites for Ubuntu 14.04

OSv is currently failing to compile in a freshly installed Ubuntu 14.04.  The
reason is that we're missing the g++-multilib package, even after the setup
script is run.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>