kbuild: make KBUILD_NOCMDDEP=1 handle empty built-in.o

Based on a patch by Rabin Vincent.

Fix building with KBUILD_NOCMDDEP=1, which currently does not work
because it does not build built-in.o with no dependencies:

  LD      fs/notify/built-in.o
ld: cannot find fs/notify/dnotify/built-in.o: No such file or directory
ld: cannot find fs/notify/inotify/built-in.o: No such file or directory
ld: cannot find fs/notify/fanotify/built-in.o: No such file or directory

Reported-and-tested-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Michal Marek <mmarek@suse.cz>