affile, afsocket: Properly call the LogSrcDriver queue method

Since both the affile and afsocket sources are descendants of
LogSrcDriver, they both should call log_src_driver_queue_method(), so
that it can set the SOURCE macro appropriately, and do its other tasks.

The socket source was explicitly setting the queue method to NULL, for a
fallback to log_pipe_forward_msg(), while the file source was setting
its own properties, and failed to call the parents queue method.

The other sources were unaffected. This closes #186.

Reported-by: Fabien Wernli
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>