kbuild: fix building vmlinux.o

Ingo Molnar wrote:
>
> i've got a build log from a weird build error below:
>
>   LD      init/built-in.o
> distcc[12023] ERROR: compile (null) on localhost failed
> make: *** [vmlinux.o] Error 1
> make: *** Waiting for unfinished jobs....
>   LD      .tmp_vmlinux1
>

Building vmlinux.o were moved up in the dependency chain so we started
to build it before the kallsym stuff. This was done to let modpost
report section mismatch bugs even when the final link failed.

Originally I had expected the dependency of $(kallsyms.o) to
cover this but it turns out that we need to be even more explicit.
Fix this by adding a conditional dependency on firat target
used in the kallsyms serie of builds.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Roland McGrath <roland@redhat.com>