Arduino_LED_Matrix: add const to matrixWrite

This allows you to then change the sketch(s) used within the test apps
for the Q to use this header file with little changes to them.

Like the Weather forecast on LED matrix
```
#include "Arduino_LED_Matrix.h"
Arduino_LED_Matrix matrix;

void setup() {
  matrix.begin();
...
```

Or in my test app I also then included ArduinoGraphics
``
#include <Arduino_RouterBridge.h>
#include "ArduinoGraphics.h"
...

Author: KurtE

libraries/Arduino_LED_Matrix/src/Arduino_LED_Matrix.h

@@ -7,7 +7,7 @@ void matrixEnd(void);
 void matrixPlay(uint8_t *buf, uint32_t len);
 void matrixSetGrayscaleBits(uint8_t _max);
 void matrixGrayscaleWrite(uint8_t *buf);
-void matrixWrite(uint32_t *buf);
+void matrixWrite(const uint32_t *buf);
 };
 
 #if __has_include("ArduinoGraphics.h")