network: Undefine existing gai_strerror definitions

network: Undefine existing gai_strerror definitions

This avoids warnings if there already exists a definition.

This is the case on windows, where the getaddrinfo isn't available
and linked to (and we use our fallbacks instead, which actually
try to use the proper getaddrinfo version if found at runtime),
but gai_strerror still exists as a define.

Signed-off-by: Martin Storsjö <martin@martin.st>