base: Add Allocator_avl::size_at function

This function provides a way to request the size of an previously
allocated block. It is useful to to ease the implementation of realloc
functionality based on Allocator_avl.

base/include/base/allocator_avl.h

@@ -231,6 +231,11 @@ namespace Genode {
 
 			void free(void *addr, size_t) { free(addr); }
 
+			/**
+			 * Return size of block at specified address
+			 */
+			size_t size_at(void const *addr) const;
+
 			/**
 			 * Return the memory overhead per Block
 			 *