Remove various compilation warnings

With the removal of these warnings we slightly improve the robustness of
the highscore parsing, handling some more cases of bad parsing/writing
and failure to open files.

src/highscore.h

@@ -1,6 +1,8 @@
 #ifndef HIGHSCORE_H
 #define HIGHSCORE_H
 
+#include "engine.h"
+
 void highscore_reset(void);
 long highscore_load(struct gamestate *g);
 void highscore_save(struct gamestate *g);