If the user enables the tls-rust feature and forgets to disable
default-features (which includes tls-native), tell them and bail out.
The code was made to work anyway when both are enabled, and here it
defaults to tls-native. It does seem better to have one explicitely
choose one though hence the compile_error! message.
Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>
Without it, it seems impossible to start TLS on an XMPPStream object,
preventing any use of that object on its own (not wrapped into
AsyncClient or somesuch).
