powerpc/mm: fix redundant inclusion of pgtable-frag.o in Makefile

The patch identified below added pgtable-frag.o to obj-y
but some merge witchery kept it also for obj-CONFIG_PPC_BOOK3S_64

This patch clears the duplication.

Fixes: 737b434d ("powerpc/mm: convert Book3E 64 to pte_fragment")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>