[flang][NFC] Fix mis-matched struct/class declarations

The template `ListDirectedStatementState` was declared as a struct and then as a class.
Fix it so they match.

Differential Revision: https://reviews.llvm.org/D88711

Author: tskeith

flang/runtime/io-stmt.h

@@ -149,10 +149,11 @@ struct IoStatementBase : public DefaultFormatControlCallbacks {
 };
 
 // Common state for list-directed internal & external I/O
-template <Direction> struct ListDirectedStatementState;
+template <Direction> class ListDirectedStatementState;
 template <>
-struct ListDirectedStatementState<Direction::Output>
+class ListDirectedStatementState<Direction::Output>
     : public FormattedIoStatementState {
+public:
   static std::size_t RemainingSpaceInRecord(const ConnectionState &);
   bool NeedAdvance(const ConnectionState &, std::size_t) const;
   bool EmitLeadingSpaceOrAdvance(