Silence a warning due to stub methods not having return statements

src/kaleidoscope/device/Base.h

@@ -74,8 +74,8 @@ class Base {
    public:
     NoOpSerial() {}
     void begin(uint32_t baud) {}
-    size_t print(...) {}
+    size_t print(...) { return 0; }
-    size_t println(...) {}
+    size_t println(...) { return 0; }
     int peek() {
       return 0;
     }