Fix #39355 load images.lib.php before image_format_supported in API (#39462)

The POST /documents endpoint with generateThumbs=1 called
image_format_supported() after requiring only files.lib.php, but that
function is defined in images.lib.php, which was never included on this
path. The call raised a fatal error and the request returned a bare
HTTP 500 with no JSON body. Add the missing require_once.

Signed-off-by: Dolicraft <contact@dolicraft.com>
Co-authored-by: Dolicraft <contact@dolicraft.com>

htdocs/api/class/api_documents.class.php

@@ -1061,6 +1061,7 @@ class Documents extends DolibarrApi
 
 		if (is_object($object) && $generateThumbs) {
 			require_once DOL_DOCUMENT_ROOT.'/core/lib/files.lib.php';
+			require_once DOL_DOCUMENT_ROOT.'/core/lib/images.lib.php';	// image_format_supported() is defined here
 			if (image_format_supported($dest_file)) {
 				$object->addThumbs($dest_file);
 			}