This fixes the following failure when doing a clean build (in particular
no .ninja* lying around) of lib/libMLIRControlFlowToSCF.a only:
```
In file included from llvm/include/llvm/IR/Module.h:22,
from mlir/include/mlir/Dialect/LLVMIR/LLVMDialect.h:37,
from mlir/lib/Conversion/ControlFlowToSCF/ControlFlowToSCF.cpp:19
llvm/include/llvm/IR/Attributes.h:90:14: fatal error: llvm/IR/Attributes.inc: No such file or directory
```
bbc0e631d2