powerpc32: add support for csum_add()

[Jochen Rollwagen]

This patch

https://lists.ozlabs.org/pipermail/linuxppc-dev/2014-September/121144.html

only compiles after putting an #ifndef ARCH_HAS_CSUM_ADD around the 
definition in include/net/checksum.h

This is missing from the patch
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