go.net/ipv4: Disable an example.

This is an example that isn't executed normally anyway
because the function has arguments. Adding an underscore prefix
makes it easier to exclude it from automated tools.

R=adg, r, dave
CC=golang-dev
https://golang.org/cl/6570062

ipv4/example_test.go

@@ -222,7 +222,7 @@ func ExampleIPOSPFListener() {
 	}
 }
 
-func ExampleWriteIPOSPFHello(c *ipv4.RawConn, ifs []*net.Interface) {
+func _ExampleWriteIPOSPFHello(c *ipv4.RawConn, ifs []*net.Interface) {
 	hello := make([]byte, OSPFHelloHeaderLen)
 
 	ospf := make([]byte, OSPFHeaderLen)