[flang] Fix build failure (NFC)

llvm-project/flang/include/flang/Lower/SymbolMap.h:52:1:
error: 'SymbolBox' 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 SymbolBox : public fir::details::matcher<SymbolBox> {
^
llvm-project/flang/include/flang/Lower/AbstractConverter.h:56:1: note: did you mean struct here?
class SymbolBox;
^~~~~
struct