Add as many values as there are fields in the list

This wasn't causing trouble, but it looks more correct this way.

libgnucash/engine/gnc-engine.c

@@ -77,7 +77,7 @@ gnc_engine_init_part2()
         { "dbi", "gncmod-backend-dbi", TRUE },
 #endif
         { "xml", "gncmod-backend-xml", TRUE },
-        { NULL, FALSE }
+        { NULL, NULL, FALSE }
     }, *lib;
 
     for (lib = libs; lib->lib ; lib++)