metag: move EXPORT_SYMBOL(csum_partial) to metag_ksyms.c

Move EXPORT_SYMBOL(csum_partial) from lib/checksum.c into metag_ksyms.c
so that it doesn't get omitted by the static linker if it's not used by
any other statically linked code, which can result in undefined symbols
when building modules.

For example a randconfig caused the following error:
ERROR: "csum_partial" [fs/reiserfs/reiserfs.ko] undefined!

Signed-off-by: James Hogan <james.hogan@imgtec.com>