powerpc/xmon: Fix build when 4xx=y and 44x=n

dump_tlb_44x() is only defined when 44x=y, but the ifdef in xmon.c
checks for 4xx, leading to a build failure:

  arch/powerpc/xmon/xmon.c:912:4: error: implicit declaration of function 'dump_tlb_44x'

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>