Export the AF_INET/AF_INET6 constants

These are needed to ensure that htools uses the same numeric values as
Python.

By the way, I'm not sure what's the best option: importing socket into
constants (this patch), or adding custom exports to
autotools/convert-constants.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>