selftests/filesystems:fix build error in overlayfs

One build issue comes up due to both mount.h included dev_in_maps.c

In file included from dev_in_maps.c:10:
/usr/include/sys/mount.h:35:3: error: expected identifier before numeric constant
   35 |   MS_RDONLY = 1,  /* Mount read-only.  */
      |   ^~~~~~~~~
In file included from dev_in_maps.c:13:

Remove one of them to solve conflict, another error comes up:

dev_in_maps.c:170:6: error: implicit declaration of function ‘mount’ [-Werror=implicit-function-declaration]
  170 |  if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) == -1) {
      |      ^~~~~
cc1: all warnings being treated as errors

and then , add sys_mount definition to solve it
After both above, dev_in_maps.c can be built correctly on my mache(gcc 10.2,glibc-2.32,kernel-5.10)

Signed-off-by: Hu Yadi <hu.yadi@h3c.com>
Link: https://lore.kernel.org/r/20240112074059.29673-1-hu.yadi@h3c.com


Acked-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>