Fix VirtualBox modules build for 486

In Tails 0.14~rc2, /lib/modules/3.2.0-4-486/updates/dkms/ is empty. Only /lib/modules/3.2.0-4-686-pae/updates/dkms/ has VirtualBox guest modules.

Feature Branch: bugfix/fix_virtualbox_dkms_build

Original created by @tails on 5509 (Redmine)

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