mount/mount06.c: fix a compile error

The definitions of MS_PRIVATE and MS_MOVE were only added to glibc headers
in version 2.12.

If there are no definitions of these two flags, make it.

Signed-off-by: DAN LI <li.dan@cn.fujitsu.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>