do not include pthread when compiling without threading support

This fixes the following error, when compiling for systems without
pthread support, and when passing WITH_THREADING=no to make:

    thread_mosq.c:24:12: fatal error: pthread.h: No such file or directory
     #  include <pthread.h>
                 ^~~~~~~~~~~
    compilation terminated.

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>