perf tools: Use just forward declarations for struct thread where possible

Removing various instances of unnecessary includes, reducing the maze of
header dependencies.

Link: http://lkml.kernel.org/n/tip-hwu6eyuok9pc57alookyzmsf@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/session.h b/tools/perf/util/session.h
index 1ffae42..47b5e7d 100644
--- a/tools/perf/util/session.h
+++ b/tools/perf/util/session.h
@@ -5,8 +5,6 @@
 #include "event.h"
 #include "header.h"
 #include "machine.h"
-#include "symbol.h"
-#include "thread.h"
 #include "data.h"
 #include "ordered-events.h"
 #include <linux/kernel.h>
@@ -14,6 +12,7 @@
 #include <linux/perf_event.h>
 
 struct ip_callchain;
+struct symbol;
 struct thread;
 
 struct auxtrace;