class_shared.m4: Decorate private class prototype if `_WRAP_GOBJECT` has a function decoration specified (master branch)

Hi,

When attempting to add Visual Studio build support for gtksourceviewmm, I ran into a linker error where the symbol "static void Gtk::Object_Class::class_init_function()" could not be located, so it seems to me that we were actually supposed to export the classes that are defined in the <mm_package_includedir>/<mm_package>/private/*.h headers, contrary to what I thought.

To make things complete, I updated class_shared.m4 to decorate the generated class definitions in the *_p.h headers if one passes in a function decorator (for example, GLIBMM_API) when using _WRAP_GOBJECT, so that these classes can also be exported in a Visual Studio (or clang-cl) build as well.

With blessings, thank you!