inet_ntop: guard extra work by IF_LINT

inet_ntop: guard extra work by IF_LINT

No need to penalize the code just to silence a gcc warning, so
follow the conventions used elsewhere in gnulib.  Clients that
want to use extra warnings should also be using -Dlint.

* lib/inet_ntop.c (inet_ntop6): Mark spurious initialization, for
better code generation when not checking for warnings.
Suggested by Paul Eggert and Jim Meyering.

Signed-off-by: Eric Blake <eblake@redhat.com>