[flang] Fix an unused variable warning

This patch fixes:

  flang/lib/Lower/OpenACC.cpp:876:14: error: unused variable
  'nbRangeArgs' [-Werror,-Wunused-variable]

Author: kazutakahirata

flang/lib/Lower/OpenACC.cpp

@@ -876,6 +876,7 @@ static void genCombiner(fir::FirOpBuilder &builder, mlir::Location loc,
     unsigned nbRangeArgs = recipe.getCombinerRegion().getArguments().size() - 2;
     assert((nbRangeArgs / 3 == seqTy.getDimension()) &&
            "Expect 3 block arguments per dimension");
+    (ovid) nbRangeArgs;
     llvm::SmallVector<fir::DoLoopOp> loops;
     llvm::SmallVector<mlir::Value> ivs;
     for (unsigned i = 2; i < recipe.getCombinerRegion().getArguments().size();