selftests/x86/xstate: Fix spelling mistake "hader" -> "header"

There is a spelling mistake in a sig_print() message. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250227091533.599213-1-colin.i.king@gmail.com