selftests/futex: Add newline to ksft_exit_fail_msg()

selftests/futex: Add newline to ksft_exit_fail_msg()

This was missed in commit e5c04d0f3ea0 ("selftests/futex: Refactor
futex_wait with kselftest_harness.h") while replacing previous perror()
calls, which automatically append the newline character.

Fixes: e5c04d0f3ea0 ("selftests/futex: Refactor futex_wait with kselftest_harness.h")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251015173556.2899646-1-cmllamas@google.com