demo/posix/cobalt: add missing __STD() annotation to sigwait()

We want to wait for ^C or any hangup condition received from the root
stage, make sure to annotate as __STD(sigwait()) to bypass symbol
wrapping.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
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