build: Fix libgettexpo linking.
* gettext-tools/libgettextpo/Makefile.am: Pass $(LTLIBUNISTRING) in
libgettextpo_la_LDFLAGS, not libgettextpo_la_LIBADD. This is needed per the
documentation
https://www.gnu.org/software/automake/manual/html_node/Libtool-Flags.html, and
also for consistency with how we handle the dependencies to libintl and
libiconv.
