Add generate_mbstring_dict.php

Forgot to git add it previously.

Author: nikic

sapi/fuzzer/generate_mbstring_dict.php

@@ -0,0 +1,12 @@
+<?php
+
+if (!extension_loaded('mbstring')) {
+    echo "Skipping mbstring dictionary generation\n";
+    return;
+}
+
+$dict = "";
+foreach (mb_list_encodings() as $encoding) {
+    $dict .= "\"" . $encoding . ",\"\n";
+}
+file_put_contents(__DIR__ . "/dict/mbstring", $dict);