Fix build on Plan 9

All tests are passing.

control_plan9.go

@@ -0,0 +1,9 @@
+package reuseport
+
+import (
+	"syscall"
+)
+
+func Control(network, address string, c syscall.RawConn) error {
+	return nil
+}

control_unix.go

@@ -1,4 +1,4 @@
-// +build !windows,!wasm
+// +build !plan9,!windows,!wasm
 
 package reuseport