Add missing inline to please gcc10
Fixes link errors like
CMakeFiles/SelAlgorithms.dir/src/CombineTracksSIMD_FittedForwardTracks.cpp.o:(.bss+0x0): multiple definition of `Sel::Utils::get_track_property_from_particle'; CMakeFiles/SelAlgorithms.dir/src/CombineTracks.cpp.o:(.bss+0x0): first defined here
See discussion in LHCb!2717 (merged). @graven @rmatev FYI