vim: add nativesdk to BBCLASSEXTEND

vim: add nativesdk to BBCLASSEXTEND

Enable nativesdk build support in order to be able to include vim in
nativesdk targets (e.g. buildtools-tarball/sdk). It is useful to be able
to provide an editor in a relocatable form for when hosts do not provide
an editor. Additionally the vim recipe provides the xxd tool.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>