Generate fd-passing arguments for document portal

Add an annotation that lets us pass an fd-list to
the generated wrapper around Add(). This is more
convenient than calling the method manually.

Currently, we have no callers for the wrapper, so
this does not affect any other code.

document-portal/Makefile.am.inc

@@ -10,7 +10,9 @@ $(xdp_dbus_built_sources) : data/org.freedesktop.portal.Documents.xml
 	$(AM_V_GEN) $(GDBUS_CODEGEN)				\
 		--interface-prefix org.freedesktop.portal.	\
 		--c-namespace XdpDbus				\
+                --c-generate-autocleanup all                    \
 		--generate-c-code $(builddir)/document-portal/xdp-dbus	\
+                --annotate "org.freedesktop.portal.Documents.Add()" org.gtk.GDBus.C.UnixFD "yes" \
 		$(srcdir)/data/org.freedesktop.portal.Documents.xml	\
 		$(NULL)