lib/rsa: avoid -Wdiscarded-qualifiers

The return type of EVP_PKEY_get0_RSA() is const struct rsa_st *.
Our code drops the const qualifier leading to

In file included from tools/lib/rsa/rsa-sign.c:1:
./tools/../lib/rsa/rsa-sign.c: In function ‘rsa_add_verify_data’:
./tools/../lib/rsa/rsa-sign.c:631:13: warning:
assignment discards ‘const’ qualifier from pointer target type
[-Wdiscarded-qualifiers]
  631 |         rsa = EVP_PKEY_get0_RSA(pkey);
      |             ^

Add a type conversion.

Signed-off-by: Heinrich Schuchardt <[email protected]>

lib/rsa/rsa-sign.c

@@ -628,7 +628,7 @@ int rsa_add_verify_data(struct image_sign_info *info, void *keydest)
 	if (ret)
 		goto err_get_pub_key;
 
-	rsa = EVP_PKEY_get0_RSA(pkey);
+	rsa = (RSA *)EVP_PKEY_get0_RSA(pkey);
 	ret = rsa_get_params(rsa, &exponent, &n0_inv, &modulus, &r_squared);
 	if (ret)
 		goto err_get_params;