ginetsocketaddress: Explicitly initialise flags for getaddrinfo()

This is not a functional change, as the hints field is static and hence
automatically initialised to zero — which happens to be what AF_UNSPEC
and the unspecified protocol are defined as. However, it’s best to be
explicit about this, in case AF_UNSPEC is _not_ defined as zero.

https://bugzilla.gnome.org/show_bug.cgi?id=732739