lib: list_sort.c: add a blank line to avoid kernel-doc warnings

In order for a list to be recognized as such, blank lines
are required.

Solve those Sphinx warnings:

./lib/list_sort.c:162: WARNING: Unexpected indentation.
./lib/list_sort.c:163: WARNING: Block quote ends without a blank line; unexpected unindent.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>