nss: Remove RES_USE_INET6 usage

nss: Remove RES_USE_INET6 usage

The recent deprecation in glibc (commit b76e065991ec) means the
module will fail to build entirely:

  nss/libvirt_nss.c: In function '_nss_libvirt_gethostbyname_r':
  nss/libvirt_nss.c:363:13: error: RES_USE_INET6 is deprecated [-Werror]
     int af = ((_res.options & RES_USE_INET6) ? AF_INET6 : AF_INET);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This resolver option was removed shortly after being introduced,
and application using it are already broken anyway.

(cherry picked from commit 5fff7b99db42376e00dbc7af13f08ab27c8256e7)