refactoring: Removing the currentPlayer global variable

src/main/c/GameTic_Tac_Toe/game.c

@@ -60,7 +60,7 @@ int isBoardFull() {
 
 // Funktion zum Zug eines Spielers
 char switchPlayer(char currentPlayer) {
-    return (currentPlayer == 'X') ? 'O' : 'X';
+    return (currentPlayer == PLAYER_X) ? PLAYER_O : PLAYER_X;
 }
 
 int isValidMove(int choice) {