cpupower tool: makefile: simplify the recipe used to generate cpupower.pot target

Use the '-p' and '-o' switches to specify the pathname of the output
file to xgettext(1). This avoids to move manually the output file if
xgettext(1) succeeds.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>