Add attribute height to tag for retaining_wall

Same as for hedge https://github.com/openstreetmap/iD/blob/mast/data/presets/presets/barrier/hedge.json

data/presets/presets/barrier/retaining_wall.json

@@ -1,4 +1,7 @@
 {
+    "fields": [
+        "height"
+    ],
     "geometry": [
         "line",
         "area"
@@ -7,4 +10,4 @@
         "barrier": "retaining_wall"
     },
     "name": "Retaining Wall"
-}
+}