rust: kernel: use feature individually

`rustfmt` (in its default config, at least) does not add a comma
in the last feature of the list, which means diffs are bigger at times.

In addition, this is how the standard library does it, and one
may argue it allows for easier `grep`ing too.

Link: #439 (comment).

Indirectly-suggested-by: Benoit de Chezelles <benoit.dechezelles@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/lib.rs

@@ -12,24 +12,22 @@
 //! do so first instead of bypassing this crate.
 
 #![no_std]
-#![feature(
-    allocator_api,
-    associated_type_defaults,
-    concat_idents,
-    const_fn_trait_bound,
-    const_mut_refs,
-    const_ptr_offset_from,
-    const_refs_to_cell,
-    const_trait_impl,
-    doc_cfg,
-    generic_associated_types,
-    maybe_uninit_extra,
-    ptr_metadata,
-    receiver_trait,
-    coerce_unsized,
-    dispatch_from_dyn,
-    unsize
-)]
+#![feature(allocator_api)]
+#![feature(associated_type_defaults)]
+#![feature(concat_idents)]
+#![feature(const_fn_trait_bound)]
+#![feature(const_mut_refs)]
+#![feature(const_ptr_offset_from)]
+#![feature(const_refs_to_cell)]
+#![feature(const_trait_impl)]
+#![feature(doc_cfg)]
+#![feature(generic_associated_types)]
+#![feature(maybe_uninit_extra)]
+#![feature(ptr_metadata)]
+#![feature(receiver_trait)]
+#![feature(coerce_unsized)]
+#![feature(dispatch_from_dyn)]
+#![feature(unsize)]
 
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.