gk/fib.c:check_gateway_prefix() is too strict

The default/example configuration sets `10.0.1.1/24` as the front IPv4 network. Therefore, `10.0.1.2` is a neighbor. However, adding the routing `10.0.0.0/8` with gateway `10.0.1.2` fails because `gk/fib.c:check_gateway_prefix()` concludes that `DYC/1: Gateway address is in prefix 10.0.0.0/8, so gateway is not a neighbor`.