mkeficapsule: fix efi_firmware_management_capsule_header data type

The data type of item_offset_list shall be UINT64 according to the UEFI [1]
specifications.

In include/efi_api.h the correct data type is used. The bug was probably
never noticed because of little endianness.

[1] https://uefi.org/specs/UEFI/2.10/index.html



Signed-off-by: Malte Schmidt <malte.schmidt@weidmueller.com>

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>