Fix declaration of shared global variables async_*

In a header file write it with extern keyword. And in one of the *.c
file declare it without extern keyword.

sysdep/unix/io.c

@@ -2071,6 +2071,7 @@ watchdog_stop(void)
 
 volatile int async_config_flag;		/* Asynchronous reconfiguration/dump scheduled */
 volatile int async_dump_flag;
+volatile int async_shutdown_flag;
 
 void
 io_init(void)