[RISCV] Fix a warning

This patch fixes:

  llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h:256:18: error:
  unused variable 'TSFlags' [-Werror,-Wunused-variable]

llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h

diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h
index 503dab2..e75dfe3 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h
@@ -253,7 +253,7 @@
 }
 
 static inline unsigned getTKOpNum(const MCInstrDesc &Desc) {
-  const uint64_t TSFlags = Desc.TSFlags;
+  [[maybe_unused]] const uint64_t TSFlags = Desc.TSFlags;
   assert(hasTWidenOp(TSFlags) && hasTKOp(TSFlags));
   return Desc.getNumOperands() - 3;
 }