selftests/namespaces: remove unused utils.h include from listns_efault_test

Remove the inclusion of ../filesystems/utils.h from listns_efault_test.c.
The test doesn't use any symbols from that header. Including it alongside
../pidfd/pidfd.h causes a build failure because both headers define
wait_for_pid() with conflicting linkage:

  ../filesystems/utils.h:  extern int wait_for_pid(pid_t pid);
  ../pidfd/pidfd.h:        static inline int wait_for_pid(pid_t pid)

All symbols the test actually uses (create_child, read_nointr,
write_nointr, sys_pidfd_send_signal) come from pidfd.h.

Reported-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/all/acPV19IY3Gna6Ira@sirena.org.uk


Fixes: 07d7ad46 ("selftests/namespaces: test for efault")
Signed-off-by: Christian Brauner <brauner@kernel.org>