powerpc/lib: Do not include string.o in obj-y twice

In the Makefile, string.o (which is generated from string.S) is
included into the list of objects being built unconditionally
(obj-y) in line 12.

Additionally, if CONFIG_PPC64 is set, it is included again in
line 17.

This patch removes the latter unnecessary inclusion.

Signed-off-by: Andreas Ruprecht <rupran@einserver.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>