Fix GDBusNodeInfo leak in ibus_service_class_add_interfaces

A call to g_dbus_node_info_unref was in place but commented out.

This fixes:

==20039== 424 (40 direct, 384 indirect) bytes in 1 blocks are definitely lost in loss record 2,115 of 2,159
==20039==    at 0x4A081D4: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==20039==    by 0x56ECE69: g_malloc0 (gmem.c:127)
==20039==    by 0x56ED191: g_malloc0_n (gmem.c:362)
==20039==    by 0x519FBF2: parse_data_get_node (gdbusintrospection.c:1171)
==20039==    by 0x519FFE1: parser_start_element (gdbusintrospection.c:1290)
==20039==    by 0x56E9C2E: emit_start_element (gmarkup.c:1048)
==20039==    by 0x56EA7CB: g_markup_parse_context_parse (gmarkup.c:1395)
==20039==    by 0x51A1082: g_dbus_node_info_new_for_xml (gdbusintrospection.c:1797)
==20039==    by 0x4E33D38: ibus_service_class_add_interfaces (ibusservice.c:610)
==20039==    by 0x4E32C24: ibus_service_class_init (ibusservice.c:213)
==20039==    by 0x546EE85: type_class_init_Wm (gtype.c:2220)
==20039==    by 0x5470586: g_type_class_ref (gtype.c:2935)
==20039==    by 0x5470539: g_type_class_ref (gtype.c:2927)
==20039==    by 0x5470539: g_type_class_ref (gtype.c:2927)
==20039==    by 0x5454D21: g_object_new_valist (gobject.c:1906)
==20039==    by 0x54543C9: g_object_new (gobject.c:1571)
==20039==    by 0x402AF0: ibus_config_dconf_new (config.c:449)
==20039==    by 0x401D59: ibus_dconf_start (main.c:61)
==20039==    by 0x401E30: main (main.c:85)
==20039==~

BUG=http://code.google.com/p/ibus/issues/detail?id=1712

Review URL: https://codereview.appspot.com/100990043
Patch from Christophe Fergeau <cfergeau@redhat.com>.