Fix compilation on Mac OS X 10.9

This fixes the following error:
```
In file included from ../src/heap.h:41:
../src/store-buffer.h:229:9: error: private field 'heap_' is not used [-Werror,-Wunused-private-field]
  Heap* heap_;
          ^
```

This is against the 3.11 branch, but it might apply to master as well.