store: fix compile warning in gf_store_unlock()

The following warning is logged when building on Fedora 20:

  store.c:712:15: warning: ignoring return value of 'lockf', declared
  with attribute warn_unused_result [-Wunused-result]

It does not really matter if unlocking fails, close() will release the
lock in any case. But, log the error in case close() can not finish and
hangs indefinitely (is that possible?).

Change-Id: If6c832f9aec10da6c1adb761b13b58e22d38a065
BUG: 1009076
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/9078
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>