Fix build and load against linux-2.6.33.x

* lttng-event.h declared but did not implement
  lttng_add_perf_counter_to_ctx on kernel >=2.6.33, the implementation
  was in lttng-context-perf-counters.c, which was only included for
  kernel >=2.6.34.  This prevented the module from being loaded.

* on kernel 2.6.33.x, lttng-context-perf-counters.c complains about
  implicit declaration for {get,put}_online_cpus and
  {,un}register_cpu_notifier; so fix header inclusion.

Signed-off-by: Samuel Martin <smartin@aldebaran-robotics.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>