slices: add old build constraint syntax to help old go versions

When a project transitively depends on x/exp/slices,
go1.16 is confused and fails with errors like:

"//go:build comment without // +build comment"

Even when import of x/exp/slices guarded with build tags, so this
is imported only when compiling with go1.18, this error is unavoidable.
Let's wait a bit longer and help users who are stuck with old go.

Updates golang/go#51436

Change-Id: I7ba589644475f55585c11ba1f7650c5ede6bc637
Reviewed-on: https://go-review.googlesource.com/c/exp/+/448996
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>

slices/slices_race_test.go

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