tipc: define needless global scoped variable static

struct _zone *tipc_zones has local scope level and
should defined with the correct scoping.

CC: Per Liden <per.liden@nospam.ericsson.com>
Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: David S. Miller <davem@davemloft.net>