percpu: Fix arithmetic on pointer to void

Spotted by Clang:

In file included from ../../loader.cc:27:
In file included from ../../drivers/virtio-net.hh:12:
In file included from ../../bsd/sys/net/if_var.h:80:
In file included from ../../bsd/sys/sys/mbuf.h:40:
In file included from ../../bsd/porting/uma_stub.h:161:
../../include/osv/percpu.hh:35:42: error: arithmetic on a pointer to void
        return reinterpret_cast<T*>(base + offset);
                                    ~~~~ ^

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>