ACPI, APEI, EINJ: fix malformed newline escape

The pr_warn message has a malformed newline escape, add in the
missing \

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>