#10 Build fails with GCC 15 because of ISO C23

[Petr Písař]

Building fails with GCC 15:

async_io.c: In function ‘aio_walk_slots’:
async_io.c:233:8: error: too many arguments to function ‘fn’; expected 0, have 1
  233 |       (*fn)(slot);
  |       ~^~~~ ~~~~

A reason is that GCC 15 moved to ISO C23 language version and that changed a meaning of an empty argument list from an unspecified list to no arguments. An attached patch fixes it together with new warning about deprecation of K&R syntax.