elfutils: towards uClibc support: disable po build

Building the po/ directory complains that the scripts in there have
been generated with gettext 0.17, while we use gettext 0.18 in
Buildroot. Since we don't care that much about po files anyway, just
disable the build of this directory.

Heavily based from work done by Stefan Fröberg, but with many further
modifications by Thomas Petazzoni.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>