[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

Author: DamonFool

flang/include/flang/Lower/AbstractConverter.h

@@ -53,7 +53,7 @@ class DerivedTypeSpec;
 
 namespace lower {
 class SymMap;
-class SymbolBox;
+struct SymbolBox;
 namespace pft {
 struct Variable;
 }