openssl: make RSA struct const

OpenSSL 3.0 broke API there, but it doesn't hurt to add the "const",
hence add it.

Fixes: #19267

src/shared/openssl-util.c

@@ -46,7 +46,7 @@ int rsa_pkey_to_suitable_key_size(
                 size_t *ret_suitable_key_size) {
 
         size_t suitable_key_size;
-        RSA *rsa;
+        const RSA *rsa;
         int bits;
 
         assert_se(pkey);