Add vendor files for go build

vendor/github.com/dlclark/regexp2/syntax/fuzz.go

@@ -0,0 +1,20 @@
+// +build gofuzz
+
+package syntax
+
+// Fuzz is the input point for go-fuzz
+func Fuzz(data []byte) int {
+	sdata := string(data)
+	tree, err := Parse(sdata, RegexOptions(0))
+	if err != nil {
+		return 0
+	}
+
+	// translate it to code
+	_, err = Write(tree)
+	if err != nil {
+		panic(err)
+	}
+
+	return 1
+}