[MLIR] Remove unneeded LLVMDialect.h include in ControlFlowToSCF.cpp

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
```

mlir/lib/Conversion/ControlFlowToSCF/ControlFlowToSCF.cpp

@@ -16,7 +16,6 @@
 #include "mlir/Dialect/ControlFlow/IR/ControlFlow.h"
 #include "mlir/Dialect/ControlFlow/IR/ControlFlowOps.h"
 #include "mlir/Dialect/Func/IR/FuncOps.h"
-#include "mlir/Dialect/LLVMIR/LLVMDialect.h"
 #include "mlir/Dialect/SCF/IR/SCF.h"
 #include "mlir/Dialect/UB/IR/UBOps.h"
 #include "mlir/Pass/Pass.h"