package/libfastjson: can use threads instead of atomics

package/libfastjson: can use threads instead of atomics

When atomic intrisics are missing, libfastjson falls back to using
pthread mutexes to manage atomicity. Of course, this is much less
efficient than atomics, but it does the job.

Propagate the new dependency to rsyslog, the sole user of libfastjson.

Note: rsyslog already depends on threads for itself, but we believe it
is better to have the exact same dependency propagated.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>