stacktrace: add forward-declaration struct task_struct

This is needed if the header is to be free-standing.
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>

include/linux/stacktrace.h

@@ -4,6 +4,8 @@
 struct task_struct;
 
 #ifdef CONFIG_STACKTRACE
+struct task_struct;
+
 struct stack_trace {
 	unsigned int nr_entries, max_entries;
 	unsigned long *entries;