posix: ensure in_addr_t and in_port_t defined via arpa/inet.h

The spec requires that `in_addr_t` and `in_port_t` are both
defined when `<arpa/inet.h>` is included.

They were added to `<netinet/in.h>` in the previous commit.

Signed-off-by: Chris Friedt <cfriedt@meta.com>