[heap.py] Add missing declaration for malloc_get_all_zones
[heap.py] Add missing declaration for malloc_get_all_zones

The evaluation context isn't guaranteed to have this declaration.

Fixes "error: use of undeclared identifier 'malloc_get_all_zones'" bugs.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@369684 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb