hardware::Atreus: Implement a dummy getLedIndex()

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/kaleidoscope/hardware/Atreus.h

@@ -54,6 +54,9 @@ class Atreus {
   cRGB getCrgbAt(uint8_t i) {
     return CRGB(0, 0, 0);
   }
+  uint8_t getLedIndex(byte row, byte col) {
+    return 0;
+  }
 
   void scanMatrix(void);
   void readMatrix(void);