sysctl: fix sparse warning: Should it be static?

Impact: Include header file.

Fix this sparse warning:
  net/core/sysctl_net_core.c:123:32: warning: symbol 'net_core_path' was not declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>