percpu: use __weak only in the definition of weak percpu variables

__weak is necessary only for definition and might even not work in
declaration.  Drop it from declaration.

This change was suggested by Ivan Kokshaysky.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>