docs-rst: add userspace API book to pdf output

The userspace API book was added without the bits required to
generate PDF output. Add them.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/conf.py

@@ -373,6 +373,8 @@ latex_documents = [
      'The kernel development community', 'manual'),
     ('sound/index', 'sound.tex', 'Linux Sound Subsystem Documentation',
      'The kernel development community', 'manual'),
+    ('userspace-api/index', 'userspace-api.tex', 'The Linux kernel user-space API guide',
+     'The kernel development community', 'manual'),
 ]
 
 # The name of an image file (relative to this directory) to place at the top of