Load plugins using include_once instead of include

The problem with using include is that when the file that is loaded
does not contain the plugin class (e.g. because the directory name is
wrong), the file could be loaded again when the plugin class is
requested a second time which will lead to a fatal error because of
class redeclaration.

inc/load.php

@@ -101,7 +101,7 @@ function load_autoload($name){
         $c = ((count($m) === 4) ? "/{$m[3]}" : '');
         $plg = DOKU_PLUGIN . "{$m[2]}/{$m[1]}$c.php";
         if(@file_exists($plg)){
-            include DOKU_PLUGIN . "{$m[2]}/{$m[1]}$c.php";
+            include_once DOKU_PLUGIN . "{$m[2]}/{$m[1]}$c.php";
         }
         return;
     }