[llvm] [cmake] FindLibXml2: fall back to xmlversion.h when pkg-config has no… (PR #207797)

[Sampath Vutkoori via llvm-commits]

https://github.com/sam-svutkoor commented:

This will fix the non-standard .pc  path installation of libxml2  

https://github.com/llvm/llvm-project/pull/207797