Fix siginterrupt namespace (bug 21597).

The siginterrupt function is XSI-shaded in POSIX.  glibc wrongly
declares it in <signal.h> for non-XSI POSIX.1:2008.  This patch fixes
the conditionals (moving it into the __USE_XOPEN_EXTENDED ||
__USE_MISC block as the minimum conservative change, although it's not
clear this obsolescent function should be declared by default).

Tested for x86_64.

	[BZ #21597]
	* signal/signal.h (siginterrupt): Make declaration conditional on
	[__USE_XOPEN_EXTENDED || __USE_MISC].