libstrophe: Add scriptlet to clone before scons

libstrophe: Add scriptlet to clone before scons

Bug: https://jira.iotivity.org/browse/IOT-1760
Change-Id: Iac4a2df91c81bc65af6742378bda2b67e66dc0cf
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21531
Reviewed-by: George Nash <george.nash@intel.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
(cherry picked from commit e1ad56040295161e2c21e48f8326631e4492e084)
Reviewed-on: https://gerrit.iotivity.org/gerrit/22085
Reviewed-by: Mats Wichmann <mats@linux.com>