kernel-headers: fix 3.0 header exports for headers with __packed

Fixes E.G. mtd/ubi-user.h, which otherwise breaks busybox 1.9 builds
with ubi applets.

Upstream post 3.0 and discussed here: https://lkml.org/lkml/2011/6/22/25



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>