doc: Unpublish wl_log* and wl_abort

The public documentation included descriptions of wl_log_stderr_handler,
wl_log_func_t wl_log_handler, wl_log and wl_abort. These are not accessible
via the public API.

Move the doxygen \endcond command to wrap these definitions, removing them
from publication.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>