fix compilation issue with ppremake

panda/src/egg/p3egg_composite1.cxx

@@ -13,6 +13,7 @@
 #include "eggFilenameNode.cxx"
 #include "eggGroup.cxx"
 #include "eggGroupNode.cxx"
+#include "eggGroupNode_ext.cxx"
 #include "eggGroupUniquifier.cxx"
 #include "eggLine.cxx"
 #include "eggMaterial.cxx"