Fix XPG4 bsd_signal namespace (bug 21552)

[Andreas Schwab]

On Jun 05 2017, Joseph Myers <joseph@codesourcery.com> wrote:

> 	[BZ #21552]
> 	* signal/signal.h (bsd_signal): Declare if [__USE_XOPEN_EXTENDED],
> 	not [__USE_XOPEN].

Ok.

Andreas.

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Andreas Schwab, schwab@linux-m68k.org
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