checkAPI: move atoi from soft-deprecated to prohibited.

checkAPI: move atoi from soft-deprecated to prohibited.

The atoi function has been completely removed from the tree.
It's still present in the GTK code, but, since GTK compilation
has been removed in the default, checkAPI target doesn't complain,
as well as the buildbot.

Change-Id: I436d6f333b99b9188734155c1f92273420354947
Reviewed-on: https://code.wireshark.org/review/18432
Petri-Dish: Dario Lombardo <lomato@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
Tested-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
Reviewed-by: Dario Lombardo <lomato@gmail.com>