libpjsip: needs threads

libpjsip: needs threads

Builds without thread support fail with the following error:

  ../src/pj/os_core_unix.c:45:21: fatal error: pthread.h: No such file or directory
   #include <pthread.h>
                       ^
  compilation terminated.

Fixes:
  http://autobuild.buildroot.org/results/744/744431e1b47d22222c4d0b48d5d20649dfeed929/
  http://autobuild.buildroot.org/results/a10/a10aca9139054502e055cb61f3925637fc515e12/

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>