base: make Allocator_avl::_avl_tree accessable

This hook allows the export of the allocator's state by a derrived
class. I.e., the final state of the allocator used for bootstrapping
core.

Ref #2092

repos/base/include/base/allocator_avl.h

@@ -193,6 +193,8 @@ class Genode::Allocator_avl_base : public Range_allocator
 
 	protected:
 
+		Avl_tree<Block> const & _block_tree() const { return _addr_tree; }
+
 		/**
 		 * Clean up the allocator and detect dangling allocations
 		 *