Register legacy particle header

Event/PhysEvent/include/Event/Particle.h

@@ -823,3 +823,6 @@ inline Gaudi::XYZVector LHCb::Particle::slopes() const {
   return Gaudi::XYZVector( fabs( m_momentum.Z() ) > 0 ? m_momentum.X() / m_momentum.Z() : 0,
                            fabs( m_momentum.Z() ) > 0 ? m_momentum.Y() / m_momentum.Z() : 0, 1.0 );
 }
+
+// Including forward declarations
+REGISTER_HEADER( LHCb::Particle, "Event/Particle.h" );