[persistent-data/space_map.h] Make destructor for space_map_detail::damage public

The compiler is unable to create a default desctructor for the derived
class missing_counts if the virtual destructor for the class damage is
private.

This fixes compilation bugs with CXXFLAGS=-std=gnu++11 together with
gcc 4.8.3 and boost 1.55.

persistent-data/space_map.h

@@ -119,6 +119,7 @@ namespace persistent_data {
 
 	namespace space_map_detail {
 		class damage {
+		public:
 			virtual ~damage() {}
 		};