docbook: fix broken media build

DocBook/v4l/ no longer has any *.png files, so the 'cp' command fails,
breaking the build.  Drop the *.png cp.

  cp: cannot stat `linux-2.6.38-git18/Documentation/DocBook/v4l/*.png': No such file or directory

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>