rpc-lib: fix printf args when printing XID

* Prior to change, XID is sometimes logged with wrong format string
* Incorrect (0x%ux): generates output of "XID: 0x1920499352x"
* Correct (0x%x): generates output of "XID: 0x72787e98"

Change-Id: Id60b673a4356a4815cdb67303612181ac5624fe3
BUG: 960153
Signed-off-by: Michael Brown <michael@netdirect.ca>
Reviewed-on: http://review.gluster.org/4949
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>