Prevent multiple inclusion by adding include guards. (#231)

This fixes #228.

Author: aentinger

libraries/Arduino_LED_Matrix/src/Arduino_LED_Matrix.h

@@ -1,3 +1,5 @@
+#pragma once
+
 #include "Arduino.h"
 #include "FspTimer.h"
 #include "gallery.h"
@@ -300,4 +302,4 @@ class ArduinoLEDMatrix
             }
         }
     }
-};
+};