selftests/mount_settattr: don't define sys_open_tree() twice

selftests/mount_settattr: don't define sys_open_tree() twice

  CC       mount_setattr_test
mount_setattr_test.c:176:19: error: redefinition of ‘sys_open_tree’
  176 | static inline int sys_open_tree(int dfd, const char *filename, unsigned int flags)
      |                   ^~~~~~~~~~~~~
In file included from mount_setattr_test.c:23:
../filesystems/overlayfs/wrappers.h:59:19: note: previous definition of ‘sys_open_tree’ with type ‘int(int,  const char *, unsigned int)’
   59 | static inline int sys_open_tree(int dfd, const char *filename, unsigned int flags)

Signed-off-by: Christian Brauner <brauner@kernel.org>