netdb: Prefer musl data types if available

netdb: Prefer musl data types if available

For better ABI compatibility and interopability, the data types that
are defined in `<lwip/netdb.h>` are replaced by the ones shipped with
the musl library (`<netdb.h>`) if available.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #48