avformat/libssh: Fix libssh defaulting to shared linkage.

Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Reviewed-by: Lukasz M <lukasz.m.luki@gmail.com>
See: [FFmpeg-devel] Fix libssh static linkage on Windows
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>