sys/net_if: fixup

missing ifdef error:
```
/sys/shell/commands/sc_net_if.c:631:27: error: ‘IPV6_MAX_ADDR_STR_LEN’ undeclared (first use in this function)
             char addr_str[IPV6_MAX_ADDR_STR_LEN];
                           ^
```

missing dependency error:
```
/home/ludwig/riot-tools/l2perf/bin/msba2/shell_commands.a(sc_net_if.o): In function `_net_if_ifconfig_ipv6_addr_convert':
/home/ludwig/RIOT/sys/shell/commands/sc_net_if.c:513: undefined reference to `inet_pton'
```
I guess I need to teach my editor to be more careful.