build.gradle: Add r8 to cloudDependencies

Fixes I463e8cdc 9cf860ce9 ("Add r8.jar to third_party and BootstrapTest", 2018-04-26).

Change-Id: I25b3229ee490527827806b50a5043bb96acb59ab
diff --git a/build.gradle b/build.gradle
index f5d54ee..c38343b 100644
--- a/build.gradle
+++ b/build.gradle
@@ -306,6 +306,7 @@
                 "ddmlib",
                 "core-lambda-stubs",
                 "openjdk/openjdk-rt-1.8",
+                "r8",
         ],
         // All dex-vms have a fixed OS of Linux, as they are only supported on Linux, and will be run in a Docker
         // container on other platforms where supported.