[crypto] Drag in configured digestInfo prefixes for any use of RSA

Ensure that the configured RSA digestInfo prefixes are included in any
build that includes rsa.o (rather than relying on x509.o or tls.o also
being present in the final binary).

This allows the RSA self-tests to be run in isolation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/rsa.c

 	.final		= rsa_final,
 	.match		= rsa_match,
 };
+
+/* Drag in objects via rsa_algorithm */
+REQUIRING_SYMBOL ( rsa_algorithm );
+
+/* Drag in crypto configuration */
+REQUIRE_OBJECT ( config_crypto );