Avoid nss function conflicts with glibc nss.h

```
[2386/3815] Compiling nsswitch/nsstest.c
../../nsswitch/nsstest.c:140:13: error: ‘nss_setpwent’ redeclared as different kind of symbol
  140 | static void nss_setpwent(void)
      |             ^~~~~~~~~~~~
In file included from ../../nsswitch/nsstest.h:53,
                 from ../../nsswitch/nsstest.c:22:
/usr/include/nss.h:186:25: note: previous declaration of ‘nss_setpwent’ was here
  186 | typedef enum nss_status nss_setpwent (int);
      |                         ^~~~~~~~~~~~
../../nsswitch/nsstest.c:155:13: error: ‘nss_endpwent’ redeclared as different kind of symbol
  155 | static void nss_endpwent(void)
      |             ^~~~~~~~~~~~
In file included from ../../nsswitch/nsstest.h:53,
                 from ../../nsswitch/nsstest.c:22:
/usr/include/nss.h:87:25: note: previous declaration of ‘nss_endpwent’ was here
   87 | typedef enum nss_status nss_endpwent (void);
      |                         ^~~~~~~~~~~~
../../nsswitch/nsstest.c:293:13: error: ‘nss_setgrent’ redeclared as different kind of symbol
  293 | static void nss_setgrent(void)
      |             ^~~~~~~~~~~~
In file included from ../../nsswitch/nsstest.h:53,
                 from ../../nsswitch/nsstest.c:22:
/usr/include/nss.h:181:25: note: previous declaration of ‘nss_setgrent’ was here
  181 | typedef enum nss_status nss_setgrent (int);
      |                         ^~~~~~~~~~~~
../../nsswitch/nsstest.c:308:13: error: ‘nss_endgrent’ redeclared as different kind of symbol
  308 | static void nss_endgrent(void)
      |             ^~~~~~~~~~~~
In file included from ../../nsswitch/nsstest.h:53,
                 from ../../nsswitch/nsstest.c:22:
/usr/include/nss.h:82:25: note: previous declaration of ‘nss_endgrent’ was here
   82 | typedef enum nss_status nss_endgrent (void);
      |                         ^~~~~~~~~~~~
```