tracing, function: Fix trace header to follow context-info option

The header display of function tracer does not follow
the context-info option, so field names are displayed even
if this option is off.

Added check for TRACE_ITER_CONTEXT_INFO trace_flags.

With following commands:
	# echo function > ./current_tracer
	# echo 0 > options/context-info
	# cat trace

This is what it looked like before:
# tracer: function
#
#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
#              | |       |          |         |
add_preempt_count <-schedule
rcu_note_context_switch <-schedule
...

This is what it looks like now:
# tracer: function
#
_raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
...

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1307113131-10045-4-git-send-email-jolsa@redhat.com


Signed-off-by: Steven Rostedt <rostedt@goodmis.org>