Fix CSRF token validation function

Security Fix:
- Changed checkToken() to dolibarr_checkToken()
- Uses the correct Dolibarr CSRF protection function
- Fixes 'Call to undefined function checkToken()' error

The form should now work properly with CSRF protection.

declarationtva_create.php

@@ -47,7 +47,7 @@ $success = '';
 
 if ($action == 'create') {
     // Check CSRF token
-    if (!checkToken()) {
+    if (!dolibarr_checkToken()) {
         $error = $langs->trans("ErrorCSRFToken");
     } elseif (!empty($start_date) && !empty($end_date)) {
         // Create the declaration with dates