net.c: Include netdb.h only if HAVE_NETDB_H is defined
net.c: Include netdb.h only if HAVE_NETDB_H is defined

Platforms that do not have getaddrinfo() most likely do not have
the netdb.h include file as well.
This change includes netdb.h only if HAVE_NETDB_H is defined
(usually by configure).

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap