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/icp/algs/modes/gcm.c

@@ -806,7 +806,7 @@ static gcm_impl_ops_t *gcm_supp_impl[ARRAY_SIZE(gcm_all_impl)];
  * fallback to the fastest generic implementation.
  */
 const gcm_impl_ops_t *
-gcm_impl_get_ops()
+gcm_impl_get_ops(void)
 {
 	if (!kfpu_allowed())
 		return (&gcm_generic_impl);

module/os/linux/spl/spl-kmem-cache.c

@@ -1420,7 +1420,7 @@ EXPORT_SYMBOL(spl_kmem_cache_reap_now);
  * it should do no harm.
  */
 int
-spl_kmem_cache_reap_active()
+spl_kmem_cache_reap_active(void)
 {
 	return (0);
 }

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);
 }