[AF_RXRPC]: AF_RXRPC depends on IPv4

Add a dependency for CONFIG_AF_RXRPC on CONFIG_INET.  This fixes this
error:

net/built-in.o: In function `rxrpc_get_peer':
(.text+0x42824): undefined reference to `ip_route_output_key'

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>