net: capture: enable capturing of IPv6/v4 only

Net capturing would not link when either IPv6 or IPv4 was switched off
due to missing symbols. This change allows for capturing a single IP
protocol only.

To reproduce:

samples/net/sockets/http_client/prj.conf
```
-CONFIG_NET_IPV4=y
+CONFIG_NET_IPV4=n
-CONFIG_NET_CONFIG_NEED_IPV4=y
+CONFIG_NET_CONFIG_NEED_IPV4=n
+
+CONFIG_NET_CAPTURE=y
```

This will cause the following linker error:
```
.../subsys/net/lib/capture/capture.c:648:
    undefined reference to `net_if_ipv4_get_ttl'
```

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>