Fix drivers/media build for modular builds

Fix allmodconfig build bug introduced in latest -git by commit
7c91f062

 ("V4L/DVB(7767): Move tuners to common/tuners"):

  LD      kernel/built-in.o
  LD      drivers/built-in.o
  ld: drivers/media/built-in.o: No such file: No such file or directory

which happens if all media drivers are modular:

  http://redhat.com/~mingo/misc/config-Wed_Apr_30_09_24_48_CEST_2008.bad

In that case there's no obj-y rule connecting all the built-in.o files and
the link tree breaks.

The fix is to add a guaranteed obj-y rule for the core vmlinux to build.
(which results in an empty object file if all media drivers are modular)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>