[flang] Fix -Wmismatched-tags warning of NonTbpDefinedIoTable (NFC)

/Users/jiefu/llvm-project/flang/runtime/non-tbp-dio.h:41:1: error: 'NonTbpDefinedIoTable' defined as a struct here but previously declared as a class; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Werror,-Wmismatched-tags]
struct NonTbpDefinedIoTable {
^
/Users/jiefu/llvm-project/flang/include/flang/Runtime/io-api.h:26:1: note: did you mean struct here?
class NonTbpDefinedIoTable;
^~~~~
struct
1 error generated.