docs-rst: add sound book to pdf output

The sound subsystem book was added without the bits required to
generate PDF output. Add them.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/sound/conf.py

@@ -0,0 +1,10 @@
+# -*- coding: utf-8; mode: python -*-
+
+project = "Linux Sound Subsystem Documentation"
+
+tags.add("subproject")
+
+latex_documents = [
+    ('index', 'sound.tex', project,
+     'The kernel development community', 'manual'),
+]