src: fix build error without OpenSSL support

PR #3890 [1] introduced the variable ALLOW_INSECURE_SERVER_DHPARAM defined
in src/node_crypto.cc. However, if nodejs is built without OpenSSL support,
the build fails:
 error: ‘ALLOW_INSECURE_SERVER_DHPARAM’ was not declared in this scope
       ALLOW_INSECURE_SERVER_DHPARAM = true;

Fix this by using the preprocessor macro HAVE_OPENSSL to opt-out the use of
ALLOW_INSECURE_SERVER_DHPARAM in non-OpenSSL builds.

[1] https://github.com/nodejs/node/pull/3890

PR-URL: https://github.com/nodejs/node/pull/4201


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>