go.mod: mark as requiring -compat 1.16

When the tagging process runs go mod tidy it needs to pass -compat 1.16.
This magic comment will cause it to do so. See CL 443857.

For golang/go#48523.

Change-Id: I6824a2c78462604b9d4737444b4d16d4c21ecc99
Reviewed-on: https://go-review.googlesource.com/c/tools/+/444295
Auto-Submit: Heschi Kreinick <heschi@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

go.mod

@@ -1,6 +1,6 @@
 module golang.org/x/tools
 
-go 1.18
+go 1.18 // tagx:compat 1.16
 
 require (
 	github.com/yuin/goldmark v1.4.13