[mlir] Update mlir/CMakeLists.txt to install *.def files

This is needed to consume mlir after it has been installed of the source
tree. Without this, consuming mlir results a build error.

Differential Revision: https://reviews.llvm.org/D72232

GitOrigin-RevId: aab72f89b19ae38ea7ac3b7b51aae68fc10aef4f