syscall: Fix TestPassFD on Solaris

<pre>: oos:syscall; go test
--- FAIL: TestPassFD (0.01 seconds)
    syscall_unix_test.go:148: expected 1 SocketControlMessage; got scms = []syscall.SocketControlMessage(nil)
FAIL
exit status 1
FAIL    syscall 0.024s</pre>
