Fix sigpause namespace (bug 21554).

The sigpause function is declared for __USE_XOPEN.  As it's new in
XPG4.2 it should be declared only for __USE_XOPEN_EXTENDED.  This
patch fixes the declaration accordingly.

Tested for x86_64.

	[BZ #21554]
	* signal/signal.h (sigpause):: Declare if [__USE_XOPEN_EXTENDED],
	not [__USE_XOPEN].