[afc] Add st_mtime support for OS 3.1
[afc] Add st_mtime support for OS 3.1

Add mtime support in g_vfs_backend_afc_set_info_from_afcinfo,
as supported by OS 3.1.

xkahn/gvfs-cmis,xkahn/gvfs-cmis,gicmo/gvfs,gicmo/gvfs,xkahn/gvfs-cmis,philipl/gvfs,philipl/gvfs,cpatulea/gvfs,halfline/gvfs,gicmo/gvfs,cpatulea/gvfs,halfline/gvfs,halfline/gvfs,gicmo/gvfs,xkahn/gvfs-cmis,philipl/gvfs