Export symbols for indirectly accessible constructors

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

src/Generator/Passes/GenerateSymbolsPass.cs

@@ -115,7 +115,6 @@ namespace CppSharp.Passes
             var method = function as Method;
             bool isInImplicitSpecialization;
             var declarationContext = function.Namespace;
-            var @class = declarationContext as Class;
             do
             {
                 isInImplicitSpecialization =

src/Generator/Passes/SymbolsCodeGenerator.cs

@@ -158,7 +158,7 @@ namespace CppSharp.Passes
             bool needSubclass = method.Access == AccessSpecifier.Protected || @class.IsAbstract;
             if (needSubclass)
             {
-                Write($"extern \"C\" void {wrapper}({signature}) ");
+                Write($"extern \"C\" {GetExporting()}void {wrapper}({signature}) ");
                 WriteLine($"{{ ::new ({Helpers.InstanceField}) {wrapper}{method.Namespace.Name}({@params}); }}");
             }
             else