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)

---
 tools/perf/util/include/linux/rbtree.h |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

index 9bcdc844b330..2a030c5af3aa 100644
@@ -1,3 +1,2 @@
 #include <stdbool.h>
-#include <stdbool.h>
 #include "../../../../include/linux/rbtree.h"

1.7.7.3