chachapoly: Use a function instead of an extern variable

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

src/libmbedcrypto.c

@@ -30,8 +30,6 @@
 #ifdef HAVE_LIBMBEDCRYPTO
 #include <mbedtls/md.h>
 
-extern const struct ssh_cipher_struct chacha20poly1305_cipher;
-
 struct ssh_mac_ctx_struct {
     enum ssh_mac_e mac_type;
     mbedtls_md_context_t ctx;
@@ -964,7 +962,7 @@ void ssh_mbedtls_init(void)
         cmp = strcmp(ssh_ciphertab[i].name, "chacha20-poly1305@openssh.com");
         if (cmp == 0) {
             memcpy(&ssh_ciphertab[i],
-                   &chacha20poly1305_cipher,
+                   ssh_get_chacha20poly1305_cipher(),
                    sizeof(struct ssh_cipher_struct));
             break;
         }