make gamemap polymorphic

src/map.cpp

@@ -325,6 +325,10 @@ gamemap::gamemap(const config& cfg, const std::string& data):
 	read(data);
 }
 
+gamemap::~gamemap()
+{
+}
+
 //! Reads a map 
 //!
 //! @param data			the mapdata to load

src/map.hpp

@@ -165,6 +165,7 @@ public:
 	//! Starting locations are represented by numbers, 
 	//! and will be of type keep.
 	gamemap(const config& terrain_cfg, const std::string& data); //throw(incorrect_format_exception)
+	virtual ~gamemap();
 	void read(const std::string& data);
 
 	std::string write() const;