Added missing header file

Author: Dr15Jones

FWCore/Framework/interface/PrincipalGetAdapter.h

@@ -107,6 +107,7 @@ edm::Ref<AppleCollection> ref(refApples, index);
 #include "FWCore/Utilities/interface/ProductLabels.h"
 #include "FWCore/Utilities/interface/propagate_const.h"
 #include "FWCore/Utilities/interface/Transition.h"
+#include "FWCore/Utilities/interface/ProductResolverIndex.h"
 
 namespace edm {