Cargo.toml: exclude contrib from workspace

This fixes e.g., `make .rav1e`:
"Error: CliError { error: Some(current package believes it's in a workspace when it's not" when building..."

See:
https://github.com/rust-lang/cargo/issues/5418

Co-authored-by: Alexandre Janniaux <ajanni@videolabs.io>

Cargo.toml

@@ -7,6 +7,7 @@ members = [
     "modules/logger/telegraf-rs/"
 ]
 resolver = "2"
+exclude = ["contrib"]
 
 [workspace.package]
 version = "4.0.0"