make ImgIoUtilReadFile use WebPMalloc instead of malloc

This is a minor technical change in API in case a special allocator
was used in WebPMalloc.

Change-Id: Idb78a9150d006f158c1a9538525097749e42e7f7

examples/img2webp.c

@@ -84,7 +84,7 @@ static int ReadImage(const char filename[], WebPPicture* const pic) {
   if (!ImgIoUtilReadFile(filename, &data, &data_size)) return 0;
   reader = WebPGuessImageReader(data, data_size);
   ok = reader(data, data_size, pic, 1, NULL);
-  free((void*)data);
+  WebPFree((void*)data);
   return ok;
 }