POSIX: change pthreads to px4_tasks in simulator

The simulator was using pthread APIs directly so calls to px4_getpid()
would fail since the task ID was not known. Changed simulator to use
px4_task_spawn_cmd.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/simulator/simulator.cpp

@@ -39,7 +39,6 @@
 #include <px4_log.h>
 #include <px4_tasks.h>
 #include <px4_time.h>
-#include <pthread.h>
 #include <poll.h>
 #include <systemlib/err.h>
 #include <errno.h>