tracing: Fix power tracer header

tracing: Fix power tracer header

Before patch:
  # tracer: power
  #
  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
  #              | |       |          |         |
  [  676.875865889] CSTATE: Going to C1 on cpu 0 for 0.005911463
  [  676.882938805] CSTATE: Going to C1 on cpu 0 for 0.104796532
  ...

After patch:
  # tracer: power
  #
  #   TIMESTAMP      STATE  EVENT
  #       |            |      |
  [  676.875865889] CSTATE: Going to C1 on cpu 0 for 0.005911463
  [  676.882938805] CSTATE: Going to C1 on cpu 0 for 0.104796532
  ...

v2: Use seq_puts instead of seq_printf

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <49E2E889.5000903@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>