Declare gethostname for XPG4 (bug 20054).

unistd.h declares gethostname for __USE_UNIX98 || __USE_XOPEN2K.  But
it's also in XPG4 (XNS volume - C438 - not the main definitions of
system interfaces and headers in C435).  This patch corrects the
condition.

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

	[BZ #20054]
	* posix/unistd.h (gethostname): Declare if [__USE_XOPEN_EXTENDED],
	not [__USE_UNIX98].
	* conform/data/unistd.h-data (gethostname): Do not expect for
	[XPG3].