BUGFIX: Fix header interdependency issue

Fixed issue where two header files included each other recursively.

player.h

 #ifndef PLAYER_HEADER
 #define PLAYER_HEADER
 
-typedef struct player {
+struct Grid;
+typedef struct Grid grid;
+
+typedef struct Player {
     int x;
     int y;
-} Player;
+} player;
 
 enum Direction {
     LEFT = 'h',
@@ -13,6 +16,6 @@ enum Direction {
     RIGHT = 'l'
 };
 
-
+void move_player(grid* G, enum Direction D);
 
 #endif