Include md5 openssl Header (#13)
Include md5 openssl Header (#13)

Motivation:

I'm switching [an AWS SDK](https://github.com/noppoMan/aws-sdk-swift) to use NIO under the hood, and there are some MD5 sum operations we need to complete related to requests + file operations.

Including [the `md5.h` header ](https://github.com/libressl/libressl/blob/master/src/crypto/md5/md5.h) will make it much easier to utilize and prevent requiring an additional package to get  at the functions already present here.

Modifications:

Included the `md5.h` header in `CNIOOpenSSL

Result:

This will allow md5 related functions to be used by packages including `CNIOOpenSSL`.
apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl