image: fit: Use stack allocation macro

The documentation above the DEFINE_ALIGN_BUFFER says it's for use
outside functions, but we're inside one.

Instead use ALLOC_CACHE_ALIGN_BUFFER, the stack based macro, which also
includes the cache alignment.

Fixes: b583348c ("image: fit: Align hash output buffers")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Tested-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>