decryptcore: update for OpenSSL 1.1 API

ERR_load_crypto_strings is deprecated in OpenSSL 1.1, and OpenSSL 1.1
generally does not require explicit initialization. However, we do need
to ensure that initialization is done before entering capability mode so
call OPENSSL_init_crypto instead. Also include header needed for
ERR_error_string.

Reviewed by: vangyzen
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40343