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.

core/data/CbmDataLinkDef.h

@@ -13,7 +13,7 @@
 #pragma link C++ class CbmDigiBranchBase + ;
 #pragma link C++ class CbmEvent + ;
 #pragma link C++ class CbmEventStore + ;
-#pragma link C++ class CbmHit;
+#pragma link C++ class CbmHit + ;
 #pragma link C++ class CbmMCTrack + ;
 #pragma link C++ class CbmPixelHit + ;
 #pragma link C++ class CbmStripHit + ;