Fix compile error on kernel 3.0.101 with CONFIG_PERF_EVENTS

The context parameter of perf_event_create_kernel_counter() was introduced
with kernel 3.1.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>