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: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/conf.py

@@ -371,6 +371,8 @@ latex_documents = [
      'The kernel development community', 'manual'),
     ('security/index', 'security.tex', 'The kernel security subsystem manual',
      'The kernel development community', 'manual'),
+    ('sound/index', 'sound.tex', 'Linux Sound Subsystem Documentation',
+     'The kernel development community', 'manual'),
 ]
 
 # The name of an image file (relative to this directory) to place at the top of

Documentation/sound/conf.py

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