Change in TSO structure not reflected in ghc-heap decoding logic

Commit 6d1700b6 introduced a new field to the TSO but didn't update the decoding login in ghc-heap (this has broken ghc-debug).

This clause was added to collect_pointers but no corresponding change to the Haskell code which calls collect_pointers.

+            if ((StgClosure *)((StgTSO *)closure)->label != NULL) {
+                ptrs[nptrs++] = (StgClosure *)((StgTSO *)closure)->label;
+            }