tools/power/x86/intel_pstate_tracer: Free the trace buffer memory

The trace buffer memory should be, mostly, freed after
the buffer has been output.

This patch is required before a future patch that will allow
the user to override the default, and specify the trace buffer
memory allocation as a command line option.

Signed-off-by: Doug Smythies <dsmythies@telus.net>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>