xbinary-io: Fix build error. * modules/xbinary-io (Depends-on): Add gettext-h. * lib/xbinary-io.c: Include gettext.h and define _(). Reported by Gisle Vanem <gisle.vanem@gmail.com> in <https://lists.gnu.org/archive/html/bug-gnulib/2017-04/msg00089.html>.