gdbus-server-auth test: Include gcredentialsprivate.h
Otherwise we'll never test the EXTERNAL-only mode, because that relies on testing the private macros G_CREDENTIALS_UNIX_CREDENTIALS_MESSAGE_SUPPORTED and G_CREDENTIALS_SOCKET_GET_CREDENTIALS_SUPPORTED.
This results in the 'out' label becoming unused on platforms like Linux where EXTERNAL authentication is supported, so #ifdef it appropriately.
Fixes: 9f962ebe "Add a test for GDBusServer authentication"