win32: call olsr_exit() instead of exit()

win32: call olsr_exit() instead of exit()

olsr_exit tries to do proper shutdown that cleans up data structures
by raise()ing the SIGTERM signal. This signal then causes olsr_shutdown
to be invoked (only when the signal handler is setup), which will do
the cleanup of the data structures.

According to the man page of raise():
  If the signal causes a handler to be called, raise() will return
  only after the signal handler has returned.

And since olsr_exit() invokes exit() after raise()ing the signal, there
is no harm in calling it.

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>