Add required use statement

Without this statement, the reaffirming of belongsToMany relations would fail. The original commit was to prove a point in an issue, but for it to work in the real world this extra bit is added.

src/Jenssegers/Mongodb/Helpers/QueriesRelationships.php

@@ -4,6 +4,7 @@ namespace Jenssegers\Mongodb\Helpers;
 
 use Closure;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
 use Illuminate\Database\Eloquent\Relations\HasOneOrMany;
 use Jenssegers\Mongodb\Eloquent\Model;