This PR adds a Findzstd.cmake script to cpptrace's install so that
`find_dependency(zstd)` can succeed on installations that don't include
zstdConfig.cmake/zstd-config.cmake.
The reason `find_dependency(zstd)` is needed in cpptrace's config,
despite `find_dependency(zstd)` also being in libdwarf's cmake and
cpptrace not needing zstd, is that libdwarf's cmake doesn't define
`zstd::libzstd_static` / `zstd::libzstd_shared` targets.
This should fix#112.
