tools/power x86_energy_perf_policy: Fix format string in error message

tools/power x86_energy_perf_policy: Fix format string in error message

The error message in validate_cpu_selected_set() uses an incomplete
format specifier "cpu%" instead of "cpu%d", resulting in the error
message printing "Requested cpu% is not present" rather than
showing the actual CPU number.

Fix the format string to properly display the CPU number.

Signed-off-by: Malaya Kumar Rout <mrout@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>