x86/i8259: Make default_legacy_pic static

The symbol is not used outside of the file, so mark it static.

Signed-off-by: Chen Lifu <chenlifu@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220823021958.3052493-1-chenlifu@huawei.com