net: add missing SetKeepAlivePeriod for Plan 9

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/12082045

src/pkg/net/tcpsock_plan9.go

@@ -65,6 +65,11 @@ func (c *TCPConn) SetKeepAlive(keepalive bool) error {
 	return syscall.EPLAN9
 }
 
+// SetKeepAlivePeriod sets period between keep alives.
+func (c *TCPConn) SetKeepAlivePeriod(d time.Duration) error {
+	return syscall.EPLAN9
+}
+
 // SetNoDelay controls whether the operating system should delay
 // packet transmission in hopes of sending fewer packets (Nagle's
 // algorithm).  The default is true (no delay), meaning that data is