We need it to build rbtree.c after this cset:

  commit d72da4a4
  Author: Peter Zijlstra <peterz@infradead.org>
  Date:   Wed May 27 11:09:36 2015 +0930

    rbtree: Make lockless searches non-fatal

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-qlnzhezv5ddwst0w9fydju0y@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>