perf report: Support forced leader feature in pipe mode

Stephane reported a problem with forced leader in pipe mode, where
report does not force the group output. The reason is that we don't
force the leader in pipe mode.

This patch adds HEADER_LAST_FEATURE mark to have a point where we have
all events and features received, and force the group if requested.

  $ perf record --group -e '{cycles, instructions}' -o - kill | perf report -i - --group

  SNIP

  #         Overhead  Command  Shared Object     Symbol
  # ................  .......  ................  .......................
  #
      28.36%   0.00%  kill     libc-2.25.so      [.] __unregister_atfork
      26.32%   0.00%  kill     libc-2.25.so      [.] _dl_addr
      26.10%   0.00%  kill     ld-2.25.so        [.] _dl_relocate_object
      17.32%   0.00%  kill     ld-2.25.so        [.] __tunables_init
       1.70%   0.01%  kill     [unknown]         [k] 0xffffffffafa01a40
       0.20%   0.00%  kill     ld-2.25.so        [.] _start
       0.00%  48.77%  kill     ld-2.25.so        [.] do_lookup_x
       0.00%  42.97%  kill     libc-2.25.so      [.] _IO_getline
       0.00%   6.35%  kill     ld-2.25.so        [.] strcmp
       0.00%   1.71%  kill     ld-2.25.so        [.] _dl_sysdep_start
       0.00%   0.19%  kill     ld-2.25.so        [.] _dl_start

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Stephane Eranian <eranian@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180314092205.23291-2-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>