python: Create macro to hide ugly function signature cast

python: Create macro to hide ugly function signature cast

-Wcast-function-type triggers various warnings related to bad api
decisions in python source. To avoid these warnings we hide them
by casting to void first. Macro here is to hide the uglyness and
provide an easy place to either/or
  + fix better in a single place in the future
  + identify the places involved should python api change in a way
    this can be handled better

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andreas Schneider <asn@samba.org>