perf ftrace: Show trace column header

perf ftrace: Show trace column header

This makes 'perf ftrace' display column header before printing trace.

  $ sudo perf ftrace
  # tracer: function
  #
  # entries-in-buffer/entries-written: 0/0   #P:8
  #
  #            TASK-PID     CPU#   TIMESTAMP  FUNCTION
  #              | |         |       |         |
             <...>-9246  [006]  10726.262760: mutex_unlock <-rb_simple_write
             <...>-9246  [006]  10726.262764: __fsnotify_parent <-vfs_write
             <...>-9246  [006]  10726.262765: fsnotify <-vfs_write
             <...>-9246  [006]  10726.262766: __sb_end_write <-vfs_write
             <...>-9246  [006]  10726.262767: fpregs_assert_state_consistent <-do_syscall_64

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200808023141.14227-6-changbin.du@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>