Declare sigwait parameter if it will be used

Make sure the 'sigw' parameter for sigwait is declared if it's going to be
used (it was missing for netbsd)
Derived from 18b932f (http://gerrit.openafs.org/2767)

Change-Id: I53f79ef5eb9ff132e99b78ca87f125e832d2e5f0
Reviewed-on: http://gerrit.openafs.org/10138
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>