UBUNTU: Add modules.builtin.bin to prerm rm list

BugLink: http://bugs.launchpad.net/bugs/516584

When purging linux-image-2.6.32-9-generic 2.6.32-9.13, dpkg would spit
out warnings about /lib/modules/2.6.32-9-generic being non-empty due to
/lib/modules/2.6.32-9-generic/modules.builtin.bin being left over.

NB: modules.builtin is actually shipped in the package, so not added to
the prerm remove list.

Signed-off-by: Loïc Minier <lool@dooz.org>
Signed-off-by: Andy Whitcroft <apw@canonical.com>