perf unwind: Provide only forward declarations for pointer types

No need to drag the headers, helps in untangling them and reducing build
time.

Link: http://lkml.kernel.org/n/tip-l8soqph92duyw5jdha0fij8b@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/unwind.h

 #ifndef __UNWIND_H
 #define __UNWIND_H
 
+#include <linux/compiler.h>
 #include <linux/types.h>
-#include "event.h"
-#include "symbol.h"
-#include "thread.h"
+
+struct map;
+struct perf_sample;
+struct symbol;
+struct thread;
 
 struct unwind_entry {
 	struct map	*map;