object: Fix CALLBACK macros with GCC 12

object: Fix CALLBACK macros with GCC 12

GCC 12 produces weird code when a callback is called directly (e.g.
parse_bool() via parse_option() in vici_config.c).  Instead of the actual
pointer, it explicitly passes 0 as first argument, which likely causes
a segmentation fault.  It doesn't happen when called indirectly via
function pointer that has void* as first argument, which this patch
kinda replicates for direct calls to avoid the issue.

Closes strongswan/strongswan#1053