perf tools: Clean target should do clean for lib/traceevent too

It's built as part of perf, so it should be cleaned too.

Tested-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1346892816-61779-1-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>