argp: do not call _IO_fwide() if _LIBC is not defined

argp: do not call _IO_fwide() if _LIBC is not defined

_IO_fwide() is defined in libio.h file. This file is included only
when _LIBC is defined.

So, in case of compilation of these files without _LIBC definition,
the compilation failed due to this unknown function.

Now this function is called when libio.h file is included.

(Change merged from gnulib.  Tested on x86_64.)

	* argp/argp-fmtstream.c (__argp_fmtstream_update): Use [_LIBC]
	conditional on calls to _IO_fwide and putwc_unlocked.  (Merge from
	gnulib.)
	* argp/argp-help.c (__argp_failure): Likewise.