[RISCV] Fix -Wbraced-scalar-init warning. NFC

Fix the following warning:
/lib/Target/RISCV/RISCVISelLowering.cpp:315:24: error: braces around scalar initializer [-Werror,-Wbraced-scalar-init]
    setOperationAction({ISD::CTLZ}, XLenVT, Legal);
                       ^~~~~~~~~~~