Build fails with clang-19: no member named '_Et' in 'ProtoJet<Item>'

```
In file included from D0RunIIConePlugin.cc:38:
In file included from ./ILConeAlgorithm.hpp:88:
./ProtoJet.hpp:198:51: error: no member named '_Et' in 'ProtoJet<Item>'
  198 |   os<<"y phi Et = ("<<_y<<", "<<_phi<<", "<<this->_Et<<")"<<std::endl;
      |                                             ~~~~  ^
1 error generated.
```

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