Fix sigevent namespace (bug 21543).

signal.h defines the sigevent structure and constants if defined
__USE_POSIX199309 || defined __USE_XOPEN_EXTENDED.  The
__USE_XOPEN_EXTENDED condition is incorrect; this structure does not
appear in XSI standards before Unix98 (which implies
__USE_POSIX199309).  This patch fixes the conditionals accordingly
(the existing conditional is correct for siginfo_t, just not for
sigevent).

Tested for x86_64.  This does not allow any conform/ XFAILs to be
removed because of other signal.h namespace bugs.

	[BZ #21543]
	* signal/signal.h: Only include <bits/types/sigevent_t.h> and
	<bits/sigevent-consts.h> if [__USE_POSIX199309], not if
	[__USE_XOPEN_EXTENDED].