nsswitch: explicitly mark NSS_STATUS _nss_winbind_* symbols as _PUBLIC_ on Linux

nsswitch: explicitly mark NSS_STATUS _nss_winbind_* symbols as _PUBLIC_ on Linux

The symbols which are used via dlopen()/dlsym() need to be exported,
in future we'll do hide all other symbols.

On other platforms, which are implemented as wrappers above the
Linux implementation, we mark the symbols as _PRIVATE_

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14780

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>