It's very simple, Check this.
T(n) = T(n-1) + $ n log n$
If you expand this then you get
T(n) = $ 1 log 1 + 2 log 2 + 3 log 3 + 4 log 4 + ........ + (n-1) log (n-1) + n log n $
T(n) <= $ n log n + n log n + n log n + n log n + ........ + n log n + n log n $
T(n) <= $ n * (n log n) $
T(n) <= $ n^2 log n $
T(n) = $ O (n^2 log n) $