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Make that explicit in the associated prototypes. This avoids a warning
on some compilers and is a correctness issue.

Corresponding to flashrom svn r449.
Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>