Ignore clippy lint warning for derive default

Rust version 1.62 made it possible to use #[derive(Default)]
(see: rust-lang/rust#94457)

As of rust version 1.68, the default clippy configuration
will print a warning for manually impl Derive on enums.
As such, users of libbpf-cargo will begin to experience clippy warnings
when they migrate to version 1.68.

libbpf-cargo's minimum rust version is 1.58, thus the recommended method
to remove the clippy warnings, is not yet an option for the project.

Add a #[allow(clippy::derivable_impls)], to suppress the clippy warning
on all generated skeletons.
Add a TODO to remove this once the project moves to a minimum rust
version of 1.62

Signed-off-by: Michael Mullin <mimullin@blackberry.com>

Author: mimullin-bbry

libbpf-cargo/src/gen.rs

@@ -670,6 +670,9 @@ fn gen_skel_attach(skel: &mut String, object: &mut BpfObj, obj_name: &str) -> Re
 fn gen_skel_contents(_debug: bool, raw_obj_name: &str, obj_file_path: &Path) -> Result<String> {
     let mut skel = String::new();
 
+    // TODO: remove the line
+    //       #[allow(clippy::derivable_impls)]
+    //       once Minimum Rust Version updated to 1.62
     write!(
         skel,
         r#"// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
@@ -683,6 +686,7 @@ fn gen_skel_contents(_debug: bool, raw_obj_name: &str, obj_file_path: &Path) ->
            #[allow(non_camel_case_types)]
            #[allow(clippy::transmute_ptr_to_ref)]
            #[allow(clippy::upper_case_acronyms)]
+           #[allow(clippy::derivable_impls)]
            mod imp {{
            use libbpf_rs::libbpf_sys;
         "#