POSIX: px4_getpid() fix

Since the PX4 code uses both px4_task and pthread APIs,
px4_getpid() must be save to call from either context.

On posix, this means we have to always return the pthread ID.

Reverted simulator change of pthread to px4_task

There may have been side effects if this was build for a target that
has process/task scoped file descriptors. It is now safe to call
px4_getpid() from this pthread context with this change for the
posix build for px4_getpid().

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

src/modules/simulator/simulator.cpp

@@ -39,6 +39,7 @@
 #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>