intl: Fix name resolution failures on kLIBC
kLIBC already has _nl_default_dirname and _nl_msg_cat_cntr in its own
gettext implmenetation.
* gettext-runtime/intl/dcigettext.c: Do not declare _nl_default_dirname
on kLIBC.
* gettext-runtime/intl/gettextP.h: Define _nl_msg_cat_cntr to
libintl_nl_msg_cat_cntr on kLIBC.
Copyright-paperwork-exempt: yes
