gunixmounts: Don't treat ZFS as a system internal fs

ZFS was originally added to the list of system internal filesystems by
commit 4cafadc955.

The rationale from https://bugzilla.gnome.org/show_bug.cgi?id=542156
doesn't seem very solid, and now we have x-gvfs-hide for this.

Also, this may contribute for trash:/// ignoring `{zfs mount path}/.Trash/`

So, remove zfs from the ignore_fs array.

Closes https://gitlab.gnome.org/GNOME/glib/issues/1599

gio/gunixmounts.c

@@ -344,7 +344,6 @@ g_unix_is_system_fs_type (const char *fs_type)
     "sysfs",
     "tmpfs",
     "usbfs",
-    "zfs",
     NULL
   };