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

module/zfs/dbuf.c

@@ -3947,7 +3947,7 @@ dmu_buf_get_user(dmu_buf_t *db_fake)
 }
 
 void
-dmu_buf_user_evict_wait()
+dmu_buf_user_evict_wait(void)
 {
 	taskq_wait(dbu_evict_taskq);
 }