curl (bundled): fix logic breakout

use --with-nss only on platforms other than Mac OS X and iOS

(see commit 4e8ab2198819551d07f303f80d2bd5f5ae53bc6e)

Change-Id: Ie6cb35ea5f5db2d08f303f70d1559188912ba8e5
Reviewed-on: https://gerrit.libreoffice.org/8418
Tested-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>