kernel.bbclass: Include randstruct seed assets in STAGING_KERNEL_BUILDDIR

When building with CONFIG_MODVERSIONS=y and CONFIG_RANDSTRUCT=y we need
to copy the build assets generated for the randstrutc seed to
STAGING_KERNEL_BUILDDIR, otherwise the out-of-tree modules build will
generate those assets which will result in a different
RANDSTRUCT_HASHED_SEED.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>