cpupower: Provide STATIC variable in Makefile for debug builds

When working on cpupower code, you often want to compile library code into the
binary.

This allows to execute modified cpupower code, even with library changes
without doing "make install"

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