kbuild: fix false positive -dirty tag caused by make-kpkg

make-kpkg modifies scripts/package/Makefile and deletes
scripts/package/builddeb as part of its build process.  Ignore these
changes so the tree isn't marked as -dirty, when it is just an
artifact of make-kpkg.  (make-kpkg clean restores the files to their
original state, and these helper scripts won't affect the final
compiled kernel in any way.)

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>