build: fix distcheck with new gtk-doc

The .actions file is generated by newer gtk-doc releases, we just
treat it as e.g. the .signals or .types files.

This is also already fixed in upstream gtk-doc:
  https://gitlab.gnome.org/GNOME/gtk-doc/-/merge_requests/61

docs/reference/libmm-glib/Makefile.am

@@ -86,6 +86,7 @@ CLEANFILES += \
 	$(DOC_MODULE).interfaces \
 	$(DOC_MODULE).prerequisites \
 	$(DOC_MODULE).signals \
+	$(DOC_MODULE).actions \
 	*.stamp \
 	-rf xml html tmpl \
 	$(NULL)