json_spirit: add missing dependency on Boost::thread

json_spirit: add missing dependency on Boost::thread

This library is built with `BOOST_SPIRIT_THREADSAFE` and thus it
requires linking with Boost::thread.

This mistake usually had no effect because other build targets had
this dependency already, but this is fragile and depends on the linker
command parameter order.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>