Add //sky back to root BUILD.gn

This accidentally got clobbered in a merge.

R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/772423007
diff --git a/BUILD.gn b/BUILD.gn
index 8a57ce3..5660b42 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -10,6 +10,7 @@
     "//mojo",
     "//services",
     "//shell",
+    "//sky",
   ]
 
   if (!is_android && !is_win) {