[mlir] Fix -Wctad-maybe-unsupported in MemRefUtils.cpp (NFC)

/Users/jiefu/llvm-project/mlir/lib/Dialect/MemRef/Utils/MemRefUtils.cpp:56:3: error: 'SmallVector' may not intend to support class template argument deduction [-Werror,-Wctad-maybe-unsupported]
  SmallVector indicesVec = llvm::to_vector(indices);
  ^
/Users/jiefu/llvm-project/mlir/include/mlir/Support/LLVM.h:69:7: note: add a deduction guide to suppress this warning
class SmallVector;
      ^
1 error generated.