example/ivy: add a "// +build" comment in tools.go

The modules in x/mobile are still tested under Go 1.16, which emits a
vet warning for files that have "//go:build" constraint comments
without matching "// +build" comments.

This fixes a build failure observed on the Go dashboard:
https://build.golang.org/log/2925343437d0c0559bcfa9d9de740a3a1aef2dc3

Change-Id: Ic0e3ed151a2ea3262d853c4e22cf042d54230ede
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/362575
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

Author: Bryan C. Mills

example/ivy/tools.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 //go:build ignore
+// +build ignore
 
 package dummy