crypto: Avoid unused parameter warnings

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/libmbedcrypto.c

@@ -853,6 +853,7 @@ cipher_gcm_get_length(struct ssh_cipher_struct *cipher,
                       size_t len,
                       uint64_t seq)
 {
+    (void)cipher;
     (void)seq;
 
     /* The length is not encrypted: Copy it to the result buffer */