add SIP SerializableLayer support

i want to use
gopacket.SerializeLayers function
with
layers.SIP{...}
(https://github.com/google/gopacket/blob/master/layers/sip.go)

but it does not implement the needed method:
```
cannot use sipLayer (variable of type *layers.SIP) as gopacket.SerializableLayer value in argument to gopacket.SerializeLayers: *layers.SIP does not implement gopacket.SerializableLayer (missing method SerializeTo)
```

would be nice if you can add this