Add a WORKSPACE.toplevel file

The WORKSPACE file in master-with-bazel is currently synthesized by the
script. If we end up with something like
https://boringssl-review.googlesource.com/c/boringssl/+/55965, we'll
need to be able to change it more easily. Make it the same way as
BUILD.

Change-Id: I12a91e9b346f4bd49bfb43ac7e4242e6ce37054c
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/56306
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

util/WORKSPACE.toplevel

@@ -0,0 +1 @@
+workspace(name = "boringssl")