Fix gcc invalid prototype warnings

Gcc -Wall warn: 'invalid prototype'

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

module/zfs/spa_history.c

@@ -175,7 +175,7 @@ spa_history_write(spa_t *spa, void *buf, uint64_t len, spa_history_phys_t *shpp,
 }
 
 static char *
-spa_history_zone()
+spa_history_zone(void)
 {
 #ifdef _KERNEL
 	return (curproc->p_zone->zone_name);