tracing, power-trace: make it build even if the power-tracer is turned off

Impact: build fix

The 'struct power_trace' definition is needed (for the event tracer) even if
the power-tracer plugin is turned off in the .config.

Cc: Steven Rostedt <srostedt@redhat.com>
LKML-Reference: <20090306104106.GF31042@elte.hu>
Signed-off-by: Ingo Molnar <mingo@elte.hu>