Fix build warnings in Shadow demo due to snprintf and unreferenced formal parameter (#357)

Fixes build warnings by making the following changes:
- Add header file for snprintf to prevent building warning in Visual Studio.
- Add for compiler warning about unused parameters.

FreeRTOS-Plus/Demo/AWS/Device_Shadow_Windows_Simulator/Device_Shadow_Demo/DemoTasks/ShadowDemoMainExample.c

@@ -50,6 +50,7 @@
 /* Standard includes. */
 #include <stdlib.h>
 #include <string.h>
+#include <stdio.h>
 
 /* Kernel includes. */
 #include "FreeRTOS.h"
@@ -566,6 +567,9 @@ void prvShadowDemoTask( void * pvParameters )
 {
     BaseType_t demoStatus = pdPASS;
 
+    /* Remove compiler warnings about unused parameters. */
+    ( void ) pvParameters;
+
     /* A buffer containing the update document. It has static duration to prevent
      * it from being placed on the call stack. */
     static char pcUpdateDocument[ SHADOW_REPORTED_JSON_LENGTH + 1 ] = { 0 };