Fix gcc invalid prototype warnings

Gcc -Wall warn: 'invalid prototype'

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

module/zfs/spa_misc.c

@@ -1516,7 +1516,7 @@ spa_busy(void)
 }
 
 void
-spa_boot_init()
+spa_boot_init(void)
 {
 	spa_config_load();
 }