Fix gcc invalid prototype warnings

Gcc -Wall warn: 'invalid prototype'

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/arc.c

@@ -3878,7 +3878,7 @@ out:
  * Free buffers that were tagged for destruction.
  */
 static void
-l2arc_do_free_on_write()
+l2arc_do_free_on_write(void)
 {
 	list_t *buflist;
 	l2arc_data_free_t *df, *df_prev;