dtc: Add -Wredundant-decls (resend)

[Jon Loeliger]

> We are almost clean already with the -Wredundant-decls warning.  The
> only exception is a declaration for isatty() inside the flex-generated
> code.  This can be removed by using flex's "never-interactive" option,
> which we probably should be using anyway, since we never parse
> interactively in the sense that this option implies.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied