posix: netdb: ensure that EAI constants are defined

Constants like `EAI_SYSTEM` should be defined in `<netdb.h>`
according to the spec.

In Zephyr, they should be defined via appropriately
namespaced z-variants (currently `DNS_EAI_SYSTEM` and so on).

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