efi_loader: Fix flexible array member definitions

efi_loader: Fix flexible array member definitions

When a structure contains a flexible array member, it is not supposed to be
included in arrays or other structs.
Quoting the C spec [0]
"Such a structure (and any union containing, possibly recursively, a
member that is such a structure) shall not be a member of a structure
or an element of an array."

IOW efi_hii_keyboard_layout should not include
struct efi_key_descriptor descriptors[]; since we use it at the
declaration of struct efi_hii_keyboard_package.

[0] https://www.dii.uchile.cl/~daespino/files/Iso_C_1999_definition.pdf
chapter 6.7.2.1

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>