types.h: don't expose struct ustat to userspace

<linux/types.h> can't be used together with <sys/ustat.h> because they
both define struct ustat:

    $ cat test.c
    #include <sys/ustat.h>
    #include <linux/types.h>
    $ gcc -c test.c
    In file included from test.c:2:
    /usr/include/linux/types.h:165: error: redefinition of 'struct ustat'

has been reported a while ago to debian, but seems to have been
lost in cat fighting: http://bugs.debian.org/429064



Signed-off-by: maximilian attems <max@stro.at>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>