[RISCV] Fix -Wbraced-scalar-init in RISCVISelLowering.cpp (NFC)

llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:339:24:
 error: braces around scalar initializer [-Werror,-Wbraced-scalar-init]
  339 |     setOperationAction({ISD::ROTL}, XLenVT, Expand);
      |                        ^~~~~~~~~~~
1 error generated.