src/extractors/CMakeLists.txt - link to libssh on Mac (homebrew exiv2)

On Mac with homebrew, exiv2 is built against libssh. I can't find a way to disable that, other than to ask the homebrew people not do that.

Therefore, on Mac only (for now) we also need to link against libssh (and find the corresponding libssh headers at compile time). If libssh isn't found by CMake then maybe exiv2 wasn't built against libssh. or someone removed libssh after the fact.