fix: add Init method on backward compat

I broke the API eventho I didn't meant to, I thought I was safe because the gorelease bot didn't reported anything but it seems to be wrong:
#33 (comment)

> gocompat says:
> ```
> Your branch is up to date with 'origin/master'.
> ```

asn.go

@@ -89,3 +89,5 @@ func (backwardCompat) AsnForIPv6(ip net.IP) (string, error) {
 	}
 	return strconv.FormatUint(uint64(asn), 10), nil
 }
+
+func (backwardCompat) Init() {}