server: free strduped port numbers

Although the leak is negligible, the clean heap on exit will ease
valgrind testing.

Change-Id: I3a7a9c8e8dc7557aa51d0b9caa244537e5e7007d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4410
Tested-by: jenkins

src/server/telnet_server.h

@@ -64,5 +64,6 @@ struct telnet_service {
 
 int telnet_init(char *banner);
 int telnet_register_commands(struct command_context *command_context);
+void telnet_service_free(void);
 
 #endif /* OPENOCD_SERVER_TELNET_SERVER_H */