alpha: remove __init annotation from exported page_is_ram()

EXPORT_SYMBOL and __init is a bad combination because the .init.text
section is freed up after the initialization.

Commit c5a13032 ("ACPI/APEI: Add parameter check before error
injection") exported page_is_ram(), hence the __init annotation should
be removed.

This fixes the modpost warning in ARCH=alpha builds:

  WARNING: modpost: vmlinux: page_is_ram: EXPORT_SYMBOL used for init symbol. Remove __init or EXPORT_SYMBOL.

Fixes: c5a13032

 ("ACPI/APEI: Add parameter check before error injection")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>