[mlir] Fix -Wunused-variable in OpenMPToLLVMIRTranslation.cpp (NFC)

/llvm-project/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp:3921:12:
 error: unused variable 'varType' [-Werror,-Wunused-variable]
      Type varType = mapInfoOp.getVarType();
           ^
1 error generated.

mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp

@@ -3918,7 +3918,7 @@ convertOmpTarget(Operation &opInst, llvm::IRBuilderBase &builder,
       // So, we don't store it in any datastructure. Instead, we just
       // do some sanity checks on it right now.
       auto mapInfoOp = mappedValue.getDefiningOp<omp::MapInfoOp>();
-      Type varType = mapInfoOp.getVarType();
+      [[maybe_unused]] Type varType = mapInfoOp.getVarType();
 
       // Check #1: Check that the type of the private variable matches
       // the type of the variable being mapped.