Bind path to xcrunwrapper in workspace files.

--
MOS_MIGRATED_REVID=120558192

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 05c5117..83c2388 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -31,3 +31,6 @@
 # scripts/workspace_user.sh and uncomment the next two lines.
 # load("/WORKSPACE.user", "android_repositories")
 # android_repositories()
+
+# This allows rules written in skylark to locate apple build tools.
+bind(name = "xcrunwrapper", actual = "@bazel_tools//tools/objc:xcrunwrapper")