lib: make table_compute_checksum() arguments const

table_compute_checksum() does neither changes the content of the
checksummed buffer nor the buffer length. Adding const to the
definition makes the function wider usable.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>