README doctests: fix safelog

Add some dummy definitions that support the example.

crates/safelog/README.md

@@ -35,6 +35,9 @@ You can disable safe-logging globally (across all threads) or locally
 (across a single thread).
 
 ```rust
+# let debug_mode = true;
+# let log_encrypted_data = |_|();
+# let big_secret = ();
 use safelog::{disable_safe_logging, with_safe_logging_suppressed};
 
 // If we're running in debug mode, turn off safe logging