powerpc/lib: fix redundant inclusion of quad.o

quad.o is only for PPC64, and already included in obj64-y,
so it doesn't have to be in obj-y

Fixes: 31bfdb03

 ("powerpc: Use instruction emulation infrastructure to handle alignment faults")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>