docs: index.rst: don't use genindex for pdf output

The genindex logic is meant to be used only for html output, as
pdf build has its own way to generate indexes.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Vinod Koul <vkoul@kernel.org> # dmaengine and soundwire
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -49,7 +49,7 @@ Interfaces for kernel debugging
    debug-objects
    tracepoint
 
-.. only::  subproject
+.. only:: subproject and html
 
    Indices
    =======

Documentation/driver-api/dmaengine/index.rst

@@ -47,7 +47,7 @@ This book adds some notes about PXA DMA
 
    pxa_dma
 
-.. only::  subproject
+.. only::  subproject and html
 
    Indices
    =======

Documentation/driver-api/soundwire/index.rst

@@ -10,7 +10,7 @@ SoundWire Documentation
    error_handling
    locking
 
-.. only::  subproject
+.. only::  subproject and html
 
    Indices
    =======

Documentation/networking/device_drivers/index.rst

@@ -24,7 +24,7 @@ Contents:
    google/gve
    mellanox/mlx5
 
-.. only::  subproject
+.. only::  subproject and html
 
    Indices
    =======

Documentation/networking/index.rst

@@ -31,7 +31,7 @@ Contents:
    tls
    tls-offload
 
-.. only::  subproject
+.. only::  subproject and html
 
    Indices
    =======

Documentation/sound/index.rst

@@ -12,7 +12,7 @@ Linux Sound Subsystem Documentation
    hd-audio/index
    cards/index
 
-.. only::  subproject
+.. only::  subproject and html
 
    Indices
    =======