Also define off_t in stdio.h for UNIX98.

Similar to my previous fix for XOPEN2K
<https://sourceware.org/ml/libc-alpha/2016-04/msg00631.html>, now that
bugs in the conformtest expectations for stdio.h for UNIX98 have been
corrected, that case too fails because fseeko and ftello are now
correctly expected, but off_t is not defined.  As in that fix, it
seems appropriate to define off_t in stdio.h for this standard as
well, and this patch does so.

Tested for x86_64 and x86 (testsuite, and that installed shared
libraries are unchanged by the patch).

	* libio/stdio.h (off_t): Also define if [__USE_UNIX98].
	[__USE_LARGEFILE64] (off64_t): Likewise.
	* conform/Makefile (test-xfail-UNIX98/stdio.h/conform): Remove
	variable.