[flang][nfc] Fix GCC 11 build

After merging https://reviews.llvm.org/D120801, Flang no longer builds
with GCC 11:
```
../llvm-project/flang/lib/Semantics/runtime-type-info.cpp:385:22: error: variable ‘lenParam’ set but not used [-Werror=unused-but-set-variable]
  385 |       for (SymbolRef lenParam : *lenParameters) {
      |                      ^~~~~~~~
```

I'm sending this without a review as a quick fix.

flang/lib/Semantics/runtime-type-info.cpp

@@ -383,6 +383,7 @@ const Symbol *RuntimeTableBuilder::DescribeType(Scope &dtScope) {
       // Create dummy deferred values for the length parameters so that the
       // DerivedTypeSpec is complete and can be used in helpers.
       for (SymbolRef lenParam : *lenParameters) {
+        (void)lenParam;
         derived.AddRawParamValue(
             std::nullopt, ParamValue::Deferred(common::TypeParamAttr::Len));
       }