kernel/async: remove redundant declaration.

It's in linux/init.h, and I'm about to change it to a bool.

Cc: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/async.c

@@ -78,8 +78,6 @@ static DECLARE_WAIT_QUEUE_HEAD(async_done);
 
 static atomic_t entry_count;
 
-extern int initcall_debug;
-
 
 /*
  * MUST be called with the lock held!