fix: Missing include in EigenStepper.hpp

[paulgessinger]

Self explanatory. This doesn't give a compilation error because we never compile the Eigen Stepper header on it's own.

[Corentin-Allaire]

Thanks for finding this !

[codecov]

Codecov Report

Merging #1117 (dabc904) into main (67afb40) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1117   +/-   ##
=======================================
  Coverage   48.56%   48.56%           
=======================================
  Files         341      341           
  Lines       17493    17493           
  Branches     8245     8245           
=======================================
  Hits         8495     8495           
  Misses       3237     3237           
  Partials     5761     5761           

Impacted Files	Coverage Δ
Core/include/Acts/Propagator/EigenStepper.hpp	67.56% <ø> (ø)

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