[llvm] [llvm] Fix incorrect usage of `LIBXML2_INCLUDE_DIRS` in the Windows release script (PR #95781)

[Alexandre Ganea via llvm-commits]

aganea wrote:

@CarlosAlbertoEnciso Can you please take a look? It was this comment https://reviews.llvm.org/D129571#3651781 that triggered the change to `LIBXML2_INCLUDE_DIRS`.

https://github.com/llvm/llvm-project/pull/95781