cargo: autodiscover should be off for examples

Ensure that cargo does not attempt to autodiscover examples. This causes
problems with utils.rs and ensures that an example section exists in the
Cargo.toml.

Cargo.toml

@@ -10,6 +10,9 @@ readme = "README.md"
 keywords = ["ip", "tcp", "udp", "ethernet", "network"]
 categories = ["embedded"]
 license = "0BSD"
+# Each example should have an explicit `[[example]]` section here to
+# ensure that the correct features are enabled.
+autoexamples = false
 
 [dependencies]
 managed = { version = "0.5", default-features = false, features = ["map"] }