Ensure that _GNU_SOURCE is defined for NI_MAXHOST and NI_MAXSERV

Ensure that _GNU_SOURCE is defined for NI_MAXHOST and NI_MAXSERV

Since glibc 2.8, these defines like `NI_MAXHOST` are exposed only
if suitable feature test macros are defined, namely: _GNU_SOURCE,
_DEFAULT_SOURCE (since glibc 2.19), or _BSD_SOURCE or _SVID_SOURCE
(before glibc 2.19), see GETNAMEINFO(3).

CLA: trivial
Fixes #13049

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/13054)