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>

tools/perf/util/build-id.c

@@ -15,6 +15,7 @@
 #include "build-id.h"
 #include "event.h"
 #include "symbol.h"
+#include "thread.h"
 #include <linux/kernel.h>
 #include "debug.h"
 #include "session.h"