crypto/sha256 must be imported

Otherwise, go-digest panics (panic: sha256 not available).

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

content/local/content_local_fuzzer.go

@@ -23,6 +23,7 @@ import (
 	"bufio"
 	"bytes"
 	"context"
+	_ "crypto/sha256"
 	"io"
 	"testing"