ext/openssl/extconf.rb: avoid -Werror=deprecated-declarations

It fails to build on Solaris:

https://rubyci.org/logs/rubyci.s3.amazonaws.com/solaris11-gcc/ruby-master/log/20200216T090008Z.log.html.gz
```
ossl_cipher.c: 関数 ‘ossl_cipher_init’ 内:
ossl_cipher.c:228:2: エラー: ‘EVP_md5’ is deprecated [-Werror=deprecated-declarations]
  228 |  EVP_BytesToKey(EVP_CIPHER_CTX_cipher(ctx), EVP_md5(), iv,
      |  ^~~~~~~~~~~~~~
In file included from /usr/include/openssl/x509.h:73,
                 from /usr/include/openssl/x509v3.h:63,
                 from ossl.h:23,
                 from ossl_cipher.c:10:
/usr/include/openssl/evp.h:732:26: 備考: ここで宣言されています
  732 | DEPRECATED const EVP_MD *EVP_md5(void);
      |                          ^~~~~~~
```

0 files changed, 0 insertions, 0 deletions