tls: move tls_cipher_size_desc to net/tls/tls.h

It's only used in net/tls/*, no need to bloat include/net/tls.h.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/dd9fad80415e5b3575b41f56b331871038362eab.1692977948.git.sd@queasysnail.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>