Do not declare class in template instantiation; add missing header

The package contains two files, which cause Clang to abort (assert):
- `PhysicsTools/PatUtils/plugins/SmearedJetProducer.cc`
- `PhysicsTools/PatUtils/plugins/SmearedPATJetProducer.cc`

After long investigation I stopped here:

    corrJetP4 =
    std::is_base_of<class PATJetCorrExtractor, Textractor>::value ?
      jetCorrExtractor_(jet, jetCorrLabel_.label(), jetCorrEtaMax_,
&rawJetP4) :
      jetCorrExtractor_(jet, jetCorr.product(), jetCorrEtaMax_,
&rawJetP4);

`PATJetCorrExtractor` was nowhere to be found in the translation unit.
We have a class declaration in template instantion.

The patch removes such declaration and includes a header (from the same
package), which declares and defines PATJetCorrExtractor.

Resolves both Clang aborts.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #6365 from davidlt/resolve-clang-aborts-pat

Do not declare class in template instantiation; add missing header