perf evlist: Set the leader in the perf_evlist__config method

perf evlist: Set the leader in the perf_evlist__config method

Since we need to ensure the leader is set before configuring the
evsel perf_event_attrs.

Reducing the boilerplate needed by tools, helping, for instance,
'perf trace', that wasn't setting the leader.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-22shm0ptkch2kgl7rtqlligx@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>