[efi] Allow compiler to perform type checks on EFI_EVENT

[efi] Allow compiler to perform type checks on EFI_EVENT

As with EFI_HANDLE, the EFI headers define EFI_EVENT as a void
pointer, rendering EFI_EVENT compatible with a pointer to itself and
hence guaranteeing that pointer type bugs will be introduced.

Redefine EFI_EVENT as a pointer to an anonymous structure (as we
already do for EFI_HANDLE) to allow the compiler to perform type
checking as expected.

Signed-off-by: Michael Brown <mcb30@ipxe.org>