Check diagonal lines

src/main/c/GameTic_Tac_Toe/game.c

@@ -25,3 +25,7 @@ char checkWinner() {
         if (board[i][0] == board[i][1] && board[i][1] == board[i][2]) return board[i][0];
         if (board[0][i] == board[1][i] && board[1][i] == board[2][i]) return board[0][i];
     }
+     // Überprüfe diagonale Linien
+    if (board[0][0] == board[1][1] && board[1][1] == board[2][2]) return board[0][0];
+    if (board[0][2] == board[1][1] && board[1][1] == board[2][0]) return board[0][2];
+