nss-util: silence warning about deprecated RES_USE_INET6

nss-util: silence warning about deprecated RES_USE_INET6

src/nss-resolve/nss-resolve.c: In function ‘_nss_resolve_gethostbyname_r’:
src/nss-resolve/nss-resolve.c:680:13: warning: RES_USE_INET6 is deprecated
 NSS_GETHOSTBYNAME_FALLBACKS(resolve);
             ^~~~~~~~~~~~~~~~~~~~~~~~~

In glibc bz #19582, RES_USE_INET6 was deprecated. This might make sense for
clients, but they didn't take into account nss module implementations which
*must* continue to support the option. glibc internally defines
DEPRECATED_RES_USE_INET6 which can be used without emitting a warning, but
it's not exported publicly. Let's do the same, and just copy the definition
to our header.