Make `get_dh1024` compatible with OpenSSL v1.1.0
Make `get_dh1024` compatible with OpenSSL v1.1.0

Fixes the following error:

```
../../../../ext/puma_http11/mini_ssl.c: In function ‘get_dh1024’:
../../../../ext/puma_http11/mini_ssl.c:90:5: error: dereferencing pointer to incomplete type ‘DH {aka struct dh_st}’
   dh->p = BN_bin2bn(dh1024_p, sizeof(dh1024_p), NULL);
     ^~
```

These changes are based on the following patch to nginx:
<https://github.com/nginx/nginx/commit/789abf2b8cfd184555a09f7001b59e82c003c43c>
("SSL: default DH parameters compatible with OpenSSL 1.1.0").

Relevant parts of the nginx source code in their full context can be
found here:
<https://github.com/nginx/nginx/blob/release-1.10.2/src/event/ngx_event_openssl.c#L954-L980>.

For reference:

> *) Made DH and DH_METHOD opaque. The structures for managing DH objects
>    have been moved out of the public header files. New functions for managing
>    these have been added.
>    [Matt Caswell]

<https://www.openssl.org/news/cl110.txt>

Fixes #1136.

unleashed/puma,respire/puma,puma/puma,puma/puma,eileencodes/puma,unleashed/puma,eileencodes/puma,unleashed/puma,looker/puma,unleashed/puma,respire/puma,looker/puma,eileencodes/puma,puma/puma,respire/puma,respire/puma,eileencodes/puma,looker/puma,puma/puma,looker/puma