kbuild: add dependency from vmlinux to sorttable

Without this dependency it's really puzzling when we bisect for a "bad"
commit in a series of sorttable change: when "git bisect" switches to
another commit, "make" just does nothing to vmlinux.

Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>