uefi-raw: Fix MSRV build

`size_of` wasn't added to the prelude until 1.80, but the uefi-raw MSRV is
currently 1.70.

Author: nicholasbishop

uefi-raw/src/table/system.rs

@@ -6,6 +6,7 @@ use crate::table::configuration::ConfigurationTable;
 use crate::table::runtime::RuntimeServices;
 use crate::table::Header;
 use crate::{Char16, Handle};
+use core::mem::size_of;
 use core::ptr;
 
 #[derive(Clone, Debug, Eq, PartialEq)]