Fix undefined MHASH_* constant on running downgrade from php 8.0

[samsonasik]

@Kenneth-Sills looking at the laravel, it seems the MHASH_* is included magically there, so this is to ensure the constants exists on php 8.0

Fixes rectorphp/rector#8471

[samsonasik]

Before

➜  rector-847-reproduction git:(main) ✗ php80 vendor/bin/rector process index.php --dry-run
PHP Fatal error:  Uncaught Error: Undefined constant "MHASH_XXH32" in /Users/samsonasik/www/rector-847-reproduction/vendor/rector/rector/vendor/illuminate/container/Container.php:818

After

➜  rector-847-reproduction git:(main) ✗ php80 vendor/bin/rector process index.php --dry-run
 1/1 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%
1 file with changes
===================

1) index.php:0

    ---------- begin diff ----------
@@ @@
 <?php

-echo str_starts_with('a', 'a');
+echo strncmp('a', 'a', strlen('a')) === 0;
    ----------- end diff -----------

[samsonasik]

@TomasVotruba I am merging it to have faster feedback to test ;)