Makefile: generate pot under po/ instead

If there's no reason to put pot under root dir we should overwrite the
orignal pot file.

Signed-off-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Merge pull request #268 from Vdragon/fix_makefile_build_pot_path

Makefile: generate pot under po/

Signed-off-by: David Aguilar <davvid@gmail.com>