powerpc: Add missing prototypes for ppc_select() & ppc_fadvise64_64()

Add missing prototypes for ppc_select() & ppc_fadvise64_64() to header
asm-prototypes.h. Fix the following warnings (treated as errors in W=1)

  arch/powerpc/kernel/syscalls.c:87:1: error: no previous prototype for ‘ppc_select’
  arch/powerpc/kernel/syscalls.c:119:6: error: no previous prototype for ‘ppc_fadvise64_64’

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>