fix drivers/media/common/tuners/ build bug

x86.git randconfig testing found a build failure on latest -git:

 drivers/built-in.o: In function `set_type':
 tuner-core.c:(.text+0x2a9a26): undefined reference to `tea5761_attach'
 tuner-core.c:(.text+0x2a9d05): undefined reference to `tda9887_attach'
 tuner-core.c:(.text+0x2a9d51): undefined reference to `xc2028_attach'
 tuner-core.c:(.text+0x2a9e22): undefined reference to `tda829x_attach'
 tuner-core.c:(.text+0x2a9e3f): undefined reference to `microtune_attach'
 drivers/built-in.o: In function `tuner_probe':
 tuner-core.c:(.text+0x2aa18a): undefined reference to `tda829x_probe'
 tuner-core.c:(.text+0x2aa302): undefined reference to `tea5761_autodetection'

with the following config:

 http://redhat.com/~mingo/misc/config-Wed_Apr_30_10_21_40_CEST_2008.bad

the problem is caused by the drivers/media/common/tuners/ subdirectory
not being part of the kbuild hierarchy anymore, due to commit
7c91f062

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

this seems similar to the problem also reported by Mike Galbraith.

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