vlc: fix sdl support

sdl video output module includes xlib headers, so it requires
xlib_libx11 to be available.

Fixes:
  http://autobuild.buildroot.org/results/d41/d41d2f36a4384289e300a3cf91d51960df9fab00/build-end.log



[Peter: check for SDL X11 backend instead]
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>