tools/power/cpupower: fix compilation with STATIC=true

Rename duplicate sysfs_read_file into cpupower_read_sysfs and fix linking.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Thomas Renninger <trenn@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>