powerpc/compat_sys: Declare syscalls

This fixes a compile error with W=1.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210823090039.166120-3-clg@kaod.org