ACPI_PCI_ROUTING_TABLE: Replace fixed-size array with flex array member

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.

source/include/acrestyp.h

@@ -927,8 +927,10 @@ typedef struct acpi_pci_routing_table
     UINT32                          Pin;
     UINT64                          Address;        /* here for 64-bit alignment */
     UINT32                          SourceIndex;
-    char                            Source[4];      /* pad to 64 bits so sizeof() works in all cases */
-
+    union {
+                                    char Pad[4];    /* pad to 64 bits so sizeof() works in all cases */
+                                    ACPI_FLEX_ARRAY(char, Source);
+    };
 } ACPI_PCI_ROUTING_TABLE;
 
 #endif /* __ACRESTYP_H__ */