linux-headers: update to 3.18-rc5

This updates the Linux header to version 3.18-rc5, adding support for
(among other things) read-only memslots on ARM and arm64.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Message-id: 1416248898-6302-1-git-send-email-ard.biesheuvel@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>