Mark all SSL_CIPHERs as const.

This lets us put the SSL_CIPHER table in the data section. For type-checking,
make STACK_OF(SSL_CIPHER) cast everything to const SSL_CIPHER*.

Note that this will require some changes in consumers which weren't using a
const SSL_CIPHER *.

Change-Id: Iff734ac0e36f9e5c4a0f3c8411c7f727b820469c
Reviewed-on: https://boringssl-review.googlesource.com/1541
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index 49fccd8..c48be73 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -333,7 +333,7 @@
 DECLARE_STACK_OF(SRTP_PROTECTION_PROFILE)
 
 typedef int (*tls_session_ticket_ext_cb_fn)(SSL *s, const unsigned char *data, int len, void *arg);
-typedef int (*tls_session_secret_cb_fn)(SSL *s, void *secret, int *secret_len, STACK_OF(SSL_CIPHER) *peer_ciphers, SSL_CIPHER **cipher, void *arg);
+typedef int (*tls_session_secret_cb_fn)(SSL *s, void *secret, int *secret_len, STACK_OF(SSL_CIPHER) *peer_ciphers, const SSL_CIPHER **cipher, void *arg);
 
 #ifndef OPENSSL_NO_SSL_INTERN