ACPICA: acpi_pci_routing_table: Replace fixed-size array with flex array member

ACPICA commit f4a3afd78c28dede0907f47951f0b73c9a776d4e

The "Source" array is actually a dynamically sized array, but it
is defined as a fixed-size 4 byte array. This results in tripping
both compile-time and run-time bounds checkers (e.g. via either
__builtin_object_size() or -fsanitize=bounds).

To retain the padding, create a union with an unused Pad variable of
size 4, and redefine Source as a proper flexible array member.

No binary changes appear in the .text nor .data sections.

Link: https://github.com/acpica/acpica/commit/f4a3afd7


Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>