Added missing plus sign in CbmDataLinkDef

The change is necessary to allow writing std::vector<CbmHit> to the output tree. Otherwise cryptic error messages are produced by ROOT. This was discovered while transitioning from TClonesArray to std::vector<CbmTrdHit> in the TRD hitfinder.

Luckily this was already encountered and discussed by @fweig here: https://github.com/root-project/root/issues/10240

Otherwise it would probably have taken me ages to figure this out....

Maybe also of interest to @p.-a.loizeau @se.gorbunov @s.zharko

On edit: Not sure whether this change produces problems elsewhere.