fastjet: Fix build w/clang5

https://hydra.nixos.org/build/68588849/nixlog/1

libtool: compile:  clang++ -DHAVE_CONFIG_H -I. -I../../include/fastjet -O2 -Wall -g -Woverloaded-virtual -DDROP_CGAL -I. -I./siscone -I./../../include -I./siscone -c SISConeBasePlugin.cc  -fno-common -DPIC -o .libs/libSISConePlugin_la-SISConeBasePlugin.o
SISConeBasePlugin.cc:12:12: error: no matching member function for call to 'structure_of'
  return a.structure_of<UserScaleBase::StructureType>().ordering_var2()
         ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./../../include/fastjet/PseudoJet.hh:1012:60: note: candidate template ignored: substitution failure [with TransformerType = fastjet::SISConeBasePlugin::UserScaleBase::StructureType]: ISO C++ specifies that qualified reference to 'StructureType' is a constructor name rather than a type in this context, despite preceding 'typename' keyword
const typename TransformerType::StructureType & PseudoJet::structure_of() const{
      ~~~~~~~~                                             ^