libbpf-cargo: Warn on clippy::absolute_paths

Commit 4ef1505 ("Enable clippy::absolute_paths lint") wrongly added
the clippy::absolute_paths lint to the allow list in libbpf-cargo. Warn
about it instead, as had been the intention.

Signed-off-by: Daniel Müller <[email protected]>

Author: d-e-s-o

libbpf-cargo/src/main.rs

@@ -1,4 +1,5 @@
-#![allow(clippy::absolute_paths, clippy::let_unit_value)]
+#![allow(clippy::let_unit_value)]
+#![warn(clippy::absolute_paths)]
 
 use std::path::PathBuf;