Suppress a compile warning

Marlin/src/feature/power_loss_recovery.h

@@ -128,7 +128,7 @@ class PrintJobRecovery {
   #if ENABLED(DEBUG_POWER_LOSS_RECOVERY)
     static void debug(PGM_P const prefix);
   #else
-    static inline void debug(PGM_P const prefix) {}
+    static inline void debug(PGM_P const prefix) { UNUSED(prefix); }
   #endif
 
   private: