acpi: Replace struct acpi_table_slit 1-element array with flex-array

struct acpi_table_slit is used for copying System Locality Information
Table data from ACPI tables. Here `entry` is a flex array but it was
using ancient 1-element fake flexible array, which has been deprecated.
Replace it with a C99 flexible array.

Signed-off-by: Wyes Karny <wyes.karny@amd.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Fixes: df8fc4e9 ("kbuild: Enable -fstrict-flex-arrays=3")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230523161815.3083-1-wyes.karny@amd.com