nss: Reconcile conditional declaration and use of `is_nscd'

This change harmonizes the declaration and use of `is_nscd' and fixes a
build failure with the "--enable-static-nss --enable-nscd"
configuration options due to `is_nscd' being used undeclared.

The purpose of `is_nscd' is to avoid (nss <-> nscd) recursion in
dynamically linked libc (SHARED) that is nscd-aware (USE_NSCD), and so
its declaration and use should be guarded by the definition of those
macros.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>