libsysprof-analyze: read property access to frame header

Also handle swapping the data when non-native endian so that we can leave
our GMappedFile unperturbed (and therefore not have to keep those bytes as
dirty pages in process).

src/libsysprof-analyze/sysprof-document-frame-private.h

@@ -26,6 +26,11 @@
 
 G_BEGIN_DECLS
 
+struct _SysprofDocumentFrameClass
+{
+  GObjectClass parent_class;
+};
+
 SysprofDocumentFrame *sysprof_document_frame_new (GMappedFile   *mapped,
                                                   gconstpointer  data,
                                                   gboolean       is_native);