Libssh: Add HEAD

Adds a HEAD to this formula & cleans up the quotes. Still builds in the
same way so no need for any additional dependencies.

I’ve also included a single-line code comment on the Yosemite build
failures and what link to watch for updates on that; If the maintainers
don’t hate the idea I’d like to start keeping track of those
single-line build-fail notations inside formulae so people can find
them and check them easily with each new release.

Closes #33014.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>