docs: Fix gtkdoc-scanner arguments so functions are found

gtk-doc stopped finding all the functions at some point. This appears to
be related to the VALA_EXPORT macro, which is declared further up in the
file.

Explicitly ignoring that decorator makes the documentation generator
work again.

docs/reference/meson.build

@@ -9,6 +9,7 @@ gnome.gtkdoc('umockdev',
   src_dir: [meson.build_root()],
   content_files: [version_xml],
   gobject_typesfile: 'umockdev.types',
+  scan_args: ['--ignore-decorators', 'VALA_EXTERN'],
   ignore_headers: ['uevent_sender.h', 'ioctl_tree.h', 'debug.h'],
   dependencies: [glib, gobject, declare_dependency(link_with : [umockdev_lib])],
   install: true,