softether: require libopenssl

softether tries to use SSLv3 functionality as a fallback. LibreSSL
doesn't support SSLv3 anymore. Two main issues prevent a patch:

- Trying to wrap the sslv3 functionality from the source with a guard
  clause results in linking errors after compiling is done.
- There are multiple security vulnerabilities with using sslv3.
- There are multiple security issues in github pertaining to using
  sslv3.
- This project seems to not be updated very often, and the security
  issues are being ignored it seems.

For people who still want to use softether, they will have to use
libopenssl.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>