Fix an interference between libintl.h and C++ (boost) header files.

Fix an interference between libintl.h and C++ (boost) header files.

Reported by Marc Jeanmougin <marc@jeanmougin.fr> in
<https://lists.gnu.org/archive/html/bug-gettext/2021-12/msg00027.html>.

* gettext-runtime/intl/libgnuintl.in.h (libintl_fprintf, libintl_vfprintf,
libintl_printf, libintl_vprintf, libintl_sprintf, libintl_vsprintf,
libintl_snprintf, libintl_vsnprintf, libintl_asprintf, libintl_vasprintf,
libintl_fwprintf, libintl_vfwprintf, libintl_wprintf, libintl_vwprintf,
libintl_swprintf, libintl_vswprintf, libintl_newlocale, libintl_duplocale,
libintl_freelocale, libintl_setlocale): Inject also into the C++ std namespace.