Have Dependabot only update libbpf-rs

The main reason we enabled Dependabot for the repository was to keep
the libbpf-rs version we use in-sync with current upstream releases, but
by default it manages all other dependencies as well.
Configure it so that it only manages libbpf-rs.

Signed-off-by: Daniel Müller <deso@posteo.net>

Author: d-e-s-o

.github/dependabot.yml

@@ -6,5 +6,7 @@ updates:
   - package-ecosystem: cargo
     versioning-strategy: auto
     directory: examples/rust/
+    allow:
+      - dependency-name: libbpf-rs
     schedule:
       interval: daily