fix: add inline keyword to function PhotonConversion::generateChildre…

…n in PhotonConversion.hpp (#3516)

A possibily missing inline keyword on PhotonConversion::generateChildren
in PhotonConversion.hpp may lead to a multiple definition error from
cmake. Add back this keyword.

Co-authored-by: Andreas Stefl <stefl.andreas@gmail.com>

Fatras/include/ActsFatras/Physics/ElectroMagnetic/PhotonConversion.hpp

@@ -255,7 +255,7 @@ Particle::Vector3 PhotonConversion::generateChildDirection(
   return direction;
 }
 
-std::array<Particle, 2> PhotonConversion::generateChildren(
+inline std::array<Particle, 2> PhotonConversion::generateChildren(
     const Particle& photon, Scalar childEnergy,
     const Particle::Vector3& childDirection) const {
   using namespace Acts::UnitLiterals;