size-change-report.sh: Register new filename for full/real initrd (#174)

The dracut initrd image under /usr should also be covered by file size
change reports.
Add the name of the image content file that has the detailed sizes.

## How to use

With and required by flatcar/scripts#3241

## Testing done

None, this here needs to be merged first to be picked up by Jenkins

Author: pothos

size-change-report.sh

@@ -110,6 +110,9 @@ function file_from_kind {
         initrd-wtd)
             echo 'flatcar_production_image_initrd_contents_wtd.txt'
             ;;
+        realinitrd-wtd)
+            echo 'flatcar_production_image_realinitrd_contents_wtd.txt'
+            ;;
         oem-*-old)
             oemid=${kind}
             oemid=${oemid#oem-}