kbuild: Fix dependencies for final vmlinux link

The if_changed directive is useless against phony targets.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 2441e78b ("kbuild: better abstract vmlinux sequential prerequisites")
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>