DocBook: make index.html generation less verbose by default

When make htmldocs is called on non-verbose mode, it will still be
verbose with index.html generation for no good reason, printing:

	rm -rf Documentation/DocBook/index.html; echo '<h1>Linux Kernel HTML Documentation</h1>' >> Documentation/DocBook/index.html && echo '<h2>Kernel Version: 4.4.0-rc1</h2>' >> Documentation/DocBook/index.html && cat Documentation/DocBook/iio.html >> Documentation/DocBook/index.html

Instead, use the standard non-verbose mode, using:

	  HTML    Documentation/DocBook/index.html

if not called with V=1.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>