lightnvm: do not compile in debugging by default

The LightNVM module exposes a debug interface when CONFIG_NVM_DEBUG is
set. This interfaces takes a string to configure media managers and
targets. Make sure this interface is only exposed when chosen
deliberately.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>