stacktrace.h: remove duplicate declaration task_struct

There is duplicate declaration for struct task_struct.
One can be removed safely.
Signed-off-by: Himanshu Maithani <hmaithani@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

include/linux/stacktrace.h

@@ -5,8 +5,6 @@ struct task_struct;
 struct pt_regs;
 
 #ifdef CONFIG_STACKTRACE
-struct task_struct;
-
 struct stack_trace {
 	unsigned int nr_entries, max_entries;
 	unsigned long *entries;