powerpc/64s: Add missing declaration for machine_check_early_boot()

There's no declaration for machine_check_early_boot(), which leads to a
build failure with W=1. Add one.

Fixes: 2f5182cf ("powerpc/64s: early boot machine check handler")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221125132521.2167039-1-mpe@ellerman.id.au