[root6] StBFChain: Declare global pointer to chain in compiled code

It appears that the ROOT6 interpreter prefers to see the declaration of
the main StBFChain in the compiled code over the one in the `bfc.C`
macro:

```
LoadTable: .L /star-sw/StarDb/AgMLGeometry/Geometry.y2021a.C
IncrementalExecutor::executeFunction: symbol 'chain' unresolved while linking [cling interface function]!
root4star: .sl79_gcc485/OBJ/StRoot/St_db_Maker/St_db_Maker.cxx:934: virtual TDataSet* St_db_Maker::LoadTable(TDataSet*): Assertion `!ee' failed.
```

Could it be that global variables in interpreted code are not visible in
other interpreted scripts?

StRoot/StBFChain/StBFChain.cxx

@@ -42,6 +42,8 @@
 TableImpl(Bfc);
 ClassImp(StBFChain);
 
+StBFChain *chain = nullptr;
+
 //_____________________________________________________________________________
 // Hack constructor.
 /*!

StRoot/macros/bfc.C

@@ -29,7 +29,10 @@ class StMessMgr;
 #include "TAttr.h"
 #include "Rtypes.h"
 #endif
-StBFChain    *chain=0; 
+
+// 'chain' is defined in StBFChain library (see StRoot/StBFChain.cxx)
+extern StBFChain* chain;
+
 //_____________________________________________________________________
 //_________________ Prototypes _______________________________________________
 void Usage();