main: permit dead code (no warnings) when building.

I want my build to tell me if there's anything wrong with it.  Dead
Code warnings distract me from checking that.

OTOH when I'm running Clippy I want it to catch everything.  Hence
this change.

src/main.rs

@@ -1,3 +1,6 @@
+// permit dead code when not using clippy
+#![cfg_attr(not(clippy), allow(dead_code))]
+
 mod card;
 mod helper;
 mod modes;