setup.py: add missing comma

As noticed by Morgan Wang, a comma is missing in the setup.py code,
resulting in an error locating package g++-multiliblibssl-dev.

Fixes #517.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>