docs: userspace-api: add iommu.rst to the index file

There's a new uAPI doc for IOMMU. Add it to the index file.
Should address this warning:

	.../Documentation/userspace-api/iommu.rst: WARNING: document isn't included in any toctree

Fixes: d0023e3ee2 ("docs: IOMMU user API")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/cc55219a551e29848e2282cd8939a4115067234c.1603791716.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/userspace-api/index.rst

@@ -22,6 +22,7 @@ place where this information is gathered.
    spec_ctrl
    accelerators/ocxl
    ioctl/index
+   iommu
    media/index
 
 .. only::  subproject and html