tls: Type cast explicitly for OpenSSL

With the release of GCC 14.1, some previous warnings are now errors,
including the SSL_select_next_proto call in
tls_context_server_alpn_select_callback. To fix the build, add
explicitly type cast.

Signed-off-by: javex <florian.ruechel@inexplicity.de>

src/tls/openssl.c

@@ -217,7 +217,7 @@ static int tls_context_server_alpn_select_callback(SSL *ssl,
     result = SSL_TLSEXT_ERR_NOACK;
 
     if (ctx->alpn != NULL) {
-        result = SSL_select_next_proto(out, 
+        result = SSL_select_next_proto((unsigned char **) out,
                                        outlen,
                                        &ctx->alpn[1], 
                                        (unsigned int) ctx->alpn[0],