x86/idt: Make idt_descr static

Commit

  3e77abda ("x86/idt: Consolidate idt functionality")

states that idt_descr could be made static, but it did not actually make
the change. Make it static now.

Fixes: 3e77abda ("x86/idt: Consolidate idt functionality")
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200619205103.30873-1-jandryuk@gmail.com