s390/thread_info.h: fix task_struct declaration warning

Add missing forward declaration for task_struct.
The warning appears when the -Werror C compiler flag is being used.
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/thread_info.h

@@ -47,6 +47,8 @@ struct thread_info {
 	.flags		= 0,			\
 }
 
+struct task_struct;
+
 void arch_release_task_struct(struct task_struct *tsk);
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);