crypto: lib/chacha20poly1305 - define empty module exit function

With no mod_exit function, users are unable to unload the module after
use. I'm not aware of any reason why module unloading should be
prohibited for this one, so this commit simply adds an empty exit
function.

Reported-and-tested-by: John Donnelly <john.p.donnelly@oracle.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>