${exec_prefix_nativesdk} doesn't exist so use prefix_nativesdk instead.
This resolves issues for code which attepts to use target_exec_prefix.
(From OE-Core rev: cd1ac8257ed2701cbe3802870183e8e1cd3b0418)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>