cache-url requires curl

In some cases cache-url can get pulled in without curl, causing it to
fail.

Co-Authored-By: Adam Harwell <flux.adam@gmail.com>

Change-Id: Ibd66c2ca4f8cc423783555d8a99b1184f43adff2

diskimage_builder/elements/cache-url/element-deps

@@ -0,0 +1 @@
+package-installs

diskimage_builder/elements/cache-url/package-installs.yaml

@@ -0,0 +1 @@
+curl:

diskimage_builder/elements/cache-url/pkg-map

@@ -0,0 +1,7 @@
+{
+    "distro": {
+        "gentoo": {
+            "curl": "net-misc/curl"
+        }
+    }
+}