Define off_t in stdio.h for XOPEN2K.

The header conformance test for stdio.h for XOPEN2K fails because the
header does not define the off_t type, used in the expected
declarations for fseeko and ftello.

The absence of this type is not actually strictly a bug (hence no bug
report being filed in Bugzilla), since POSIX didn't require the type
to be declared in this header until the 2008 edition.  However, the
glibc convention in such cases - where the type falls under the
general *_t POSIX reservation, and so it's OK to define it for all
POSIX versions - is to make the headers self-contained in this regard
even for the older POSIX versions not requiring the type to be defined
despite including other declarations depending on the type.  Thus,
this patch adjusts the condition in the header and removes the XFAIL
(rather than adapting the expectation to work when the functions are
declared using __off_t without off_t being defined).

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

	* libio/stdio.h (off_t): Define if [__USE_XOPEN2K], not
	[__USE_XOPEN2K8].
	[__USE_LARGEFILE64] (off64_t): Likewise.
	* conform/Makefile (test-xfail-XOPEN2K/stdio.h/conform): Remove
	variable.