x86/tboot: Mark tboot static

This structure is only really used in tboot.c.  The only exception
is a single tboot_enabled check, but for that we don't need an inline
function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200428051703.1625952-1-hch@lst.de