kernel/linux: fix packed attribute in exported headers

UBI headers (and maybe others as well) are broken because the 'packed'
attribute is not sanitised when the headers are exported to userspace.

Apply the fix from upstream:
https://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=f210735fe2f17a6225432ee3d1239bcf23a8659c

(Also, buildroot does the same, BTW)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

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