cpupower tools: Update .gitignore for files created in the debug directories

The files generated by the Makefiles in the debug directories aren't listed
in the .gitignore file in the root of the cpupower tool which causes these
files to show up in the output of 'git status'.

Signed-off-by: Palmer Cox <p@lmercox.com>
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>