gboxed: Add GBookmarkFile as Boxed types

Now that we've a copy function we can define it as a boxed type, making
it fully introspectable.

docs/reference/gobject/gobject-sections.txt

@@ -417,6 +417,7 @@ G_TYPE_OPTION_GROUP
 G_TYPE_URI
 G_TYPE_TREE
 G_TYPE_PATTERN_SPEC
+G_TYPE_BOOKMARK_FILE
 
 <SUBSECTION Standard>
 G_TYPE_IS_BOXED
@@ -453,6 +454,7 @@ g_option_group_get_type
 g_uri_get_type
 g_tree_get_type
 g_pattern_spec_get_type
+g_bookmark_file_get_type
 </SECTION>
 
 <SECTION>