So that when showing multiple events annotations, we can figure out
which is which:

  # perf record -a -e instructions,cycles sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.826 MB perf.data (~36078 samples) ]
  # perf evlist
  instructions
  cycles
  # perf annotate intel_idle 2> /dev/null | head -1
   Percent |	Source code & Disassembly of vmlinux for instructions
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
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@kernel.org>
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-n1r51l329434js84qtb2c6l9@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>