kbuild: rpm-pkg: install vmlinux.bz2 unconditionally

This conditional was added by commit fc370ecf

 ("kbuild: add
vmlinux to kernel rpm").  Its git-log mentioned vmlinux.bz2 was
necessary for debugging, but did not explain why ppc64 was an
exception.  I see no problem to copy vmlinux.bz2 all the time.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>