crash on saving pixbuf docs

[Jani Monoses <jani monoses gmail com>]

Probably not the best solution but a workaround for pixbuf docs crashing on 'save copy'.
They are not the in the document_types table so their type is not found by
the for loop so assert is hit.

--- backend/ev-
document-factory.c
+++ backend/ev-document-factory.c
@@ -185,6 +185,9 @@
                }
        }

+#ifdef ENABLE_PIXBUF
+       return EV_BACKEND_PIXBUF;
+#endif
        g_assert_not_reached ();

        return 0;