InDetRecStatistics: Fix compilation with hepmc2.

Missing #include.

This merge request affects 1 package:

• InnerDetector/InDetValidation/InDetRecStatistics

This merge request affects 1 file:

• InnerDetector/InDetValidation/InDetRecStatistics/src/TrackStatHelper.cxx

Adding @goetz ,@sroe as watchers

added InnerDetector main review-pending-level-1 labels

CI Result SUCCESS (hash 236c7af0)

	Athena
externals
cmake
make
tests

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Athena: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-EL9 9792] (remote access info)

removed review-pending-level-1 label

added review-approved label

merged

mentioned in commit 07fa84f0