Fix functions without a prototype

clang-15 emits the following error message for functions without
a prototype:

fs/zfs/os/linux/spl/spl-kmem-cache.c:1423:27: error:

  a function declaration without a prototype is deprecated
  in all versions of C [-Werror,-Wstrict-prototypes]

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Aidan Harris <me@aidanharr.is>
Closes #13421