Don't remove debian subdirectory if it exists on git cleanup.

This doesn't harm if you don't build a Debian package as `git cleanup` will work
just fine if the `debian` directory does not exist. Without this patch the
`distclean` rule will also remove the `debian` directory which make it
impossible to correctly build a debian package.
Merge pull request #1916 from venthur/fix/dont-remove-debian-subdir

Don't remove debian subdirectory if it exists on git cleanup.