po/Makefile: Fix generation of messages.po

po/Makefile: Fix generation of messages.po

* Tell xgettext that we use UTF-8 encoding (this is currently optional).

* Set charset=UTF-8 in messages.po. This avoids warnings from msgmerge:
  warning: Charset "CHARSET" is not a portable encoding name.

* Use filename relative to root directory (ui/gtk.c instead of ../ui/gtk.c
  or $(SRC_PATH)/ui/gtk.c) for comments in *.po files.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>