proxyjump support uses pthread functions but libssh doesn't link to the pthread library

6d1ed76c means that libssh now uses pthread functions outside of the normal thread support, but (despite the commit message saying "fix: link to pthread") it doesn't link to the library containing those functions.

Noticed doing a test ports bulk build on OpenBSD before updating the port and hydra (which uses libssh) failed to link with "ld.lld: error: undefined reference due to --no-allow-shlib-undefined: pthread_create".