corosync: fix a compiler warning

Fix following warning:

...
cluster/corosync.c: In function ‘corosync_get_local_addr’:
cluster/corosync.c:132:22: warning: declaration of ‘sin’ shadows a global declaration [-Wshadow]
  CCLD   sheep
make[1]: Leaving directory `/home/test/sheepdog/sheep'
...

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>