scripts: no need for this directory to be a Go package

It's perfectly fine for bench_literals.go to be a main package itself,
but the scripts directory doesn't need to be a Go package

This was bothering me at times; for example, "go list ./..." would
show the scripts directory, or "go install mvdan.cc/garble/..." would
build and install a "scripts" binary as well.

The file can still be built or run directly, like either of:

    go build bench_literals.go
    go run bench_literals.go

scripts/bench_literals.go

@@ -1,6 +1,8 @@
 // This script generate benchmarks for performance analysis of individual obfuscator literals.
 // Note that only the speed of obfuscated methods is measured, initialization cost or build speed are not measured.
 
+//go:build ignore
+
 package main
 
 import (