doc-rst: include index only in subproject AND html

The index should only be included if the build of the sub-folder is done
with the html-builder (HTML output).

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index 4e5abbb..fb779e6 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -32,3 +32,11 @@
    java
    bad-memory
    basic-profiling
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
+