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 <heinrich.schuchardt@canonical.com>