Add yet another layout_us-gdm variant

Really not sure why the appearance of this needle seems to change
so much, but oh well.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

needles/gnome/layout_us-gdm-20180514.json

@@ -0,0 +1,16 @@
+{
+  "tags": [
+    "gnome_layout_us",
+    "gnome_layout_ascii"
+  ],
+  "area": [
+    {
+      "type": "match",
+      "width": 33,
+      "ypos": 7,
+      "height": 13,
+      "xpos": 853
+    }
+  ],
+  "properties": []
+}