tools/power turbostat: Move debug prints from stdout to stderr

This leaves the stdout cleaner, having only counter data. It makes it
easier for programs to parse the output of turbostat, for example
selftests.

Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>