lockdep: forward declare struct task_struct

3117df04 causes this:

In file included from arch/s390/kernel/early.c:13:
include/linux/lockdep.h:300: warning:
		"struct task_struct" declared inside parameter list
include/linux/lockdep.h:300:
		warning: its scope is only this definition or
		declaration, which is probably not what you want
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/lockdep.h

@@ -8,6 +8,8 @@
 #ifndef __LINUX_LOCKDEP_H
 #define __LINUX_LOCKDEP_H
 
+struct task_struct;
+
 #ifdef CONFIG_LOCKDEP
 
 #include <linux/linkage.h>