Fix null_terminated crate's utf dependency version

The build currently fails as follows:

  error[E0658]: use of unstable library feature 'proc_macro_is_available'
    --> /home/penberg/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-1.0.33/src/detection.rs:28:21
     |
  28 |     let available = proc_macro::is_available();
     |                     ^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: see issue #71436 <rust-lang/rust#71436> for more information
     = help: add `#![feature(proc_macro_is_available)]` to the crate attributes to enable

  For more information about this error, try `rustc --explain E0658`.
  error: could not compile `proc-macro2` due to previous error

The problem is that the "null_terminated" crate depends on the "utf"
with a relaxed "0.1" version, which causes cargo to pull a version that
is incompatible with our toolchain.

Fix up the dependency by relying on the old "0.1.5" version explicitly.

kernel/Cargo.toml

@@ -10,4 +10,5 @@ bitflags = { version = "1.0" }
 intrusive-collections = { version = "0.9.2", default-features = false, features = [ "nightly", "alloc" ]}
 null-terminated = { version = "0.3.9" }
 rlibc = { version = "1.0.0" }
+utf = { version = "0.1.5", optional = true }
 xmas-elf = { version = "0.6" }