perf: fix duplicate header inclusion

#include <stdbool.h> somehow got duplicated on its way to linus's tree
(probably as a conflict resolution as things got sent through multiple
trees)
Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/perf/util/include/linux/rbtree.h

 #include <stdbool.h>
-#include <stdbool.h>
 #include "../../../../include/linux/rbtree.h"