Add event descriptor to perf header output in pipe-mode.

After this patch:

  $ perf record -e cycles sleep 1 | perf report --header
  # ========
  # captured on: Mon Jun  5 22:52:13 2017
  # ========
  #
  # hostname : lphh20
  # os release : 4.3.5-smp-801.43.0.0
  # perf version : 4.12.rc2.g439987
  # arch : x86_64
  # nrcpus online : 72
  # nrcpus avail : 72
  # cpudesc : Intel(R) Xeon(R) CPU E5-2696 v3 @ 2.30GHz
  # cpuid : GenuineIntel,6,63,2
  # total memory : 264134144 kB
  # cmdline : /root/perf record -e cycles sleep 1
  # event : name = cycles, , size = 112, { sample_period, sample_freq } = 4000, sample_type = IP|TID|TIME|PERIOD, disabled = 1, inherit = 1, mmap = 1, comm = 1, freq = 1, enable_on_exec = 1, task = 1, sample_id_all = 1, exclude_guest = 1, mmap2 = 1, comm_exec = 1
  # CPU_TOPOLOGY info available, use -I to display
  # NUMA_TOPOLOGY info available, use -I to display
  # pmu mappings: intel_bts = 6, cpu = 4, msr = 49, uncore_cbox_10 = 36, uncore_cbox_11 = 37, uncore_cbox_12 = 38, uncore_cbox_13 = 39, uncore_cbox_14 = 40, uncore_cbox_15 = 41, uncore_cbox_16 = 42, uncore_cbox_17 = 43, software = 1, power = 7, uncore_irp = 24, uncore_pcu = 48, tracepoint = 2, uncore_imc_0 = 16, uncore_imc_1 = 17, uncore_imc_2 = 18, uncore_imc_3 = 19, uncore_imc_4 = 20, uncore_imc_5 = 21, uncore_imc_6 = 22, uncore_imc_7 = 23, uncore_qpi_0 = 8, uncore_qpi_1 = 9, uncore_cbox_0 = 26, uncore_cbox_1 = 27, uncore_cbox_2 = 28, uncore_cbox_3 = 29, uncore_cbox_4 = 30, uncore_cbox_5 = 31, uncore_cbox_6 = 32, uncore_cbox_7 = 33, uncore_cbox_8 = 34, uncore_cbox_9 = 35, uncore_r2pcie = 13, uncore_r3qpi_0 = 10, uncore_r3qpi_1 = 11, uncore_r3qpi_2 = 12, uncore_sbox_0 = 44, uncore_sbox_1 = 45, uncore_sbox_2 = 46, uncore_sbox_3 = 47, breakpoint = 5, uncore_ha_0 = 14, uncore_ha_1 = 15, uncore_ubox = 25
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.000 MB (null) ]

Prior to this patch, event was not printed.
Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Acked-by: David Ahern <dsahern@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Simon Que <sque@chromium.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20170718042549.145161-17-davidcc@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>