package/libressl: depend on threads

package/libressl: depend on threads

If the toolchain does not have threads (e.g. br-arm-full-nothread),
compilation fails:

In file included from /home/fail/br-test-pkg/br-arm-full-nothread/build/libressl-3.3.3/crypto/cryptlib.c:117:
/home/fail/br-test-pkg/br-arm-full-nothread/build/libressl-3.3.3/crypto/../include/compat/pthread.h:114:15: fatal error: pthread.h: No such file or directory
  114 | #include_next <pthread.h>
      |               ^~~~~~~~~~~
compilation terminated.

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>