[mlir] add missing include to MemRefTransformOps.h

The ODS-generated code has the following:

```
::mlir::TypedValue<::mlir::transform::OperationType> getAlloca() {
  return ::llvm::cast<::mlir::TypedValue<::mlir::transform::OperationType>>(
      *getODSOperands(0).begin());
}
```

that may require the compiler seing the definition of `OperationType` so
include the corresponding header.