Make the format_arg attribute effective also in the case _INTL_REDIRECT_INLINE.
Reported by Tom Tromey <tom@tromey.com>
in <https://lists.gnu.org/archive/html/bug-gettext/2018-07/msg00000.html>.
* gettext-runtime/intl/libgnuintl.in.h (gettext, dgettext, dcgettext,
ngettext, dngettext, dcngettext): Add attribute __format_arg__ also to the
'static inline' functions.
