msgl-check: Treat missing header errors as warnings

msgl-check: Treat missing header errors as warnings

Problem reported by Richard Hughes at
<https://lists.fedoraproject.org/pipermail/devel/2014-June/199539.html>.
After <https://savannah.gnu.org/bugs/?40262>, "msgfmt -c" reports
error on missing PO headers.  However, it turned out to be too strict
for the projects using Launchpad Translations.  Let's relax the check
for the moment and wait for one or two release cycles.
* gettext-tools/src/msgl-check.c (check_header_entry): Report missing
required headers as warning, instead of error.