[mlir] Workaround for export lib generation on Windows for `mlir_arm_sme_abi_stubs` (#73147)

Using mlir cmake in downstream project fails with error
```
CMake Error at D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRTargets.cmake:2537 (message):
  The imported target "mlir_arm_sme_abi_stubs" references the file

     "D:/projs/llvm/llvm-install/lib/mlir_arm_sme_abi_stubs.lib"

  but this file does not exist.  Possible reasons include:

  * The file was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and contained

     "D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRTargets.cmake"

  but not all the files it references.

Call Stack (most recent call first):
  D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRConfig.cmake:37 (include)
  mlir/CMakeLists.txt:5 (find_package)
```

Windows cmake needs export libaries but it seems they are only being
generated if you have at least one exported symbol.
Add export attributes to symbols.

Not sure what the best approach to fix this (probably we should just
disable this lib on windows entirely), but it fixed things for me
locally.

0 files changed, 0 insertions, 0 deletions