Eeschema: fix missing part in part list bug in netlist exporter.

Use LIB_ID for LIB_PART_LESS_THAN comparison.  This prevents parts with
library name clashes from being dropped from the part list.

eeschema/netlist_exporters/netlist_exporter.h

@@ -3,7 +3,7 @@
  *
  * Copyright (C) 1992-2013 jp.charras at wanadoo.fr
  * Copyright (C) 2013 SoftPLC Corporation, Dick Hollenbeck <dick@softplc.com>
- * Copyright (C) 1992-2017 KiCad Developers
+ * Copyright (C) 1992-2018 KiCad Developers
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -76,7 +76,7 @@ struct LIB_PART_LESS_THAN
     bool operator()( LIB_PART* const& libpart1, LIB_PART* const& libpart2 ) const
     {
         // Use case specific GetName() wxString compare
-        return libpart1->GetName().Cmp( libpart2->GetName() ) < 0;
+        return libpart1->GetLibId() < libpart2->GetLibId();
     }
 };