[POWERPC] Remove duplicate export of __div64_32.

Change 3927f2e8 moved lib/lib64.c from
lib-y to obj-y, preventing the export in ppc_ksyms.c from overriding
the one in lib, and thus causing a duplicate-export warning.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>