nss: Fix build error with --disable-nscd

nss: Fix build error with --disable-nscd

The error is as follows:
nss_module.c: In function 'module_load_nss_files':
nss_module.c:117:7: error: 'is_nscd' undeclared (first use in this function)
  117 |   if (is_nscd)
      |       ^~~~~~~
nss_module.c:117:7: note: each undeclared identifier is reported only once for each function it appears in
nss_module.c:119:51: error: 'nscd_init_cb' undeclared (first use in this function); did you mean 'nscd_init'?
  119 |       void (*cb) (size_t, struct traced_file *) = nscd_init_cb;
      |                                                   ^~~~~~~~~~~~
      |                                                   nscd_init