rpki-client: sprinkle some const

rpki-client: sprinkle some const

EVP_PKEY_get0_* were made const correct in OpenSSL 3 and now cause the
build of rpki-client to emit warnings. Of course no one is able to see
these warnings because they are hidden in all the deprecation vomit.

Makes rpki-client build cleanly against OpenSSL 3 when configured with
--with-openssl-cflags=-DOPENSSL_SUPPRESS_DEPRECATED.

ok claudio deraadt job