Fixed warning

src/main.cpp

@@ -69,7 +69,7 @@ void callback(char* topic, byte* payload, unsigned int length) {
     Serial.print("Message arrived [");
     Serial.print(topic);
     Serial.print("] ");
-    for (int i = 0; i < length; i++) {
+    for (size_t i = 0; i < length; i++) {
         Serial.print((char)payload[i]);
     }
     Serial.println();