[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

mlir/CMakeLists.txt

@@ -89,6 +89,7 @@ if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
     DESTINATION include
     COMPONENT mlir-headers
     FILES_MATCHING
+    PATTERN "*.def"
     PATTERN "*.h"
     PATTERN "*.inc"
     PATTERN "*.td"