acpica: Fix build with ACPICA 20230331 and later

ACPICA is using flexible arrays since 20230331 and it broke aarch64
build.

• acpi_iort.o ---

/usr/src/sys/arm64/acpica/acpi_iort.c:103:4: error: field 'data' with
variable sized type 'union (unnamed union at
/usr/src/sys/arm64/acpica/acpi_iort.c:98:2)' not at the end of a struct
or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]

103 |         } data;
    |           ^

Reported by: bapt
Tested by: bapt