Added C.pcap_set_rfmon in pcap.go

After line 504, func (p *InactiveHandle) SetPromisc(promisc bool) error I added a function from libpcap to set handle in "radio frequency monitor mode". - Here's the code:

// SetRFMon sets handle to be in radio frequency monitor.
// (capture packets indepenedently from network)
func (p *InactiveHandle) SetRFMon(monitor bool) error {
	var mon C.int
	if monitor {
		mon = 1
	}
	if status := C.pcap_set_rfmon(p.cptr, mon); status < 0 {
		return statusError(status)
	}
	return nil
}

This is my first pull requests, might be wrong way to do it.