修复编译错误

app/src/main/cpp/MicroPhoto.cpp

@@ -532,11 +532,11 @@ Java_com_xypower_mpapp_MicroPhotoService_sendHeartbeat(
     {
         time_t ts = time(NULL);
         int ldr = GpioControl::getLightAdc();
-        
+
         char buf[64] = { 0 };
         snprintf(buf, sizeof(buf), "%s %d\r\n", FormatLocalDateTime(ts).c_str(), ldr);
 
-        appendFile("/sdcard/com.xypower.mpapp/tmp/ldr.txt", buf, strlen(buf));
+        appendFile("/sdcard/com.xypower.mpapp/tmp/ldr.txt", (const unsigned char* )buf, strlen(buf));
 
 
     });