rpm-pkg: Always regenerate the specfile

The *pkg targets are always run, so it makes no sense to cache the
generated specfile. This also fixes build errors when the specfile
becomes out of date, without the Makefile noticing it:

$ make rpm
  works
$ echo yadadada >localversion-test
$ make rpm
  fails, because kernel.spec assumes the old kernel release string

Signed-off-by: Michal Marek <mmarek@suse.cz>