[media] DocBook/media/Makefile: Avoid make htmldocs to fail

If make is called twice like that:
	make V=1 DOCBOOKS=device-drivers.xml htmldocs

Make will fail with:

	make -f ./scripts/Makefile.build obj=scripts/basic
	rm -f .tmp_quiet_recordmcount
	make -f ./scripts/Makefile.build obj=scripts build_docproc
	make -f ./scripts/Makefile.build obj=Documentation/DocBook htmldocs
	rm -rf Documentation/DocBook/index.html; echo '<h1>Linux Kernel HTML Documentation</h1>' >> Documentation/DocBook/index.html && echo '<h2>Kernel Version: 4.2.0-rc2</h2>' >> Documentation/DocBook/index.html && cat Documentation/DocBook/device-drivers.html >> Documentation/DocBook/index.html
	cp ./Documentation/DocBook//bayer.png ./Documentation/DocBook//constraints.png ./Documentation/DocBook//crop.gif ./Documentation/DocBook//dvbstb.png ./Documentation/DocBook//fieldseq_bt.gif ./Documentation/DocBook//fieldseq_tb.gif ./Documentation/DocBook//nv12mt.gif ./Documentation/DocBook//nv12mt_example.gif ./Documentation/DocBook//pipeline.png ./Documentation/DocBook//selection.png ./Documentation/DocBook//vbi_525.gif ./Documentation/DocBook//vbi_625.gif ./Documentation/DocBook//vbi_hsync.gif ./Documentation/DocBook/media/*.svg ./Documentation/DocBook/media/v4l/*.svg ./Documentation/DocBook//media_api
	cp: target './Documentation/DocBook//media_api' is not a directory
	Documentation/DocBook/Makefile:53: recipe for target 'htmldocs' failed

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