powerpc/head_check: use stdout for error messages

Prefer stderr instead of stdout for error messages.
This is a good practice and can help CI error detecting and
reporting (0day in this case).

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210815222334.9575-1-rdunlap@infradead.org