nsswitch/libwbclient: explicitly mark all wbc* symbols as _PUBLIC_

nsswitch/libwbclient: explicitly mark all wbc* symbols as _PUBLIC_

Some private functions from wbclient_internal.h already
leaked into the ABI. With hide_symbols=True we make sure
this doesn't happen again.

Having wbcRequestResponse[Priv]() as part of the ABI helps us
in order to hide winbindd_[priv_]request_response() soon.

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>