storage/posix: Fix conditional compiling for syncfs

Change-Id: Ief22e1c0f2b5074060752d70da41ae93f1028d62
BUG: 927146
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/5381
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>