resources: Update resources.json to include x86-ubuntu-img

Added in this commit:
https://gem5-review.googlesource.com/c/public/gem5-resources/+/50307

Change-Id: I36b347b4340fb719e875ae3cdc9b5facd02de4e3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-resources/+/50567
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
diff --git a/resources.json b/resources.json
index 758cfb5..c5a1862 100644
--- a/resources.json
+++ b/resources.json
@@ -44,6 +44,16 @@
             "source" : "src/boot-exit"
         },
         {
+            "type" : "artifact",
+            "name" : "x86-ubuntu-img",
+            "documentation" : "An x86 disk image with Ubuntu 18.04.",
+            "architecture" : "X86",
+            "is_zipped" : "True",
+            "md5sum" : "90e363abf0ddf22eefa2c7c5c9391c49",
+            "url" : "http://dist.gem5.org/dist/develop/images/x86/ubuntu-18-04/x86-ubuntu.img.gz",
+            "source" : "src/x86-ubuntu"
+        },
+        {
             "type" : "group",
             "name" : "linux-kernel",
             "contents" : [