Call OnLibraryAdded for all available librarys (bug 5925)

When loading multiple plugins at once (on server start or mapchange)
OnLibraryAdded is only called for libraries which are loaded after the
current plugin. The plugin isn't informed about the libraries that were
added before its OnPluginStart forward was called.

This patch calls OnLibraryAdded for all already registered libraries the
current plugin has missed.

Currently you'd need some stupid workaround like
```sourcepawn
new bool:g_bMyLibraryLoaded;

public OnAllPluginsLoaded()
{
  if(!g_bMyLibraryLoaded && LibraryExists("mylibrary"))
    OnLibraryAdded("mylibrary");
}

public OnLibraryAdded(const String:lib[])
{
  if(StrEqual(lib, "mylibrary"))
  {
    g_bMyLibraryLoaded = true;
    // Do something..
  }
}
```