syscall: don't assume that WIFCONTINUED is defined

It's not defined on the Hurd.

Change-Id: Ic464f4b09b6d62bdbd34edcbf81480940eabddb7
Reviewed-on: https://go-review.googlesource.com/c/161963
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed