thr_posix.c: fix implicit function declaration for 'pthread_setconcurrency'

thr_posix.c: In function 'ldap_pvt_thread_set_concurrency':
thr_posix.c:96:9: error: implicit declaration of function 'pthread_setconcurrency'
  return pthread_setconcurrency( n );
         ^~~~~~~~~~~~~~~~~~~~~~
         pthread_setcanceltype

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>