IOT-1583: Updating libcoap's SConscript.

IOT-1583: Updating libcoap's SConscript.

Making changes to extlibs/libcoap/SConscript to switch
to the newest tag.

Change-Id: I31cd7be0e343e5dec2ba28ea36e09e891fcb76d7
Signed-off-by: Pawel Winogrodzki <pawelwi@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17181
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: George Nash <george.nash@intel.com>
Reviewed-by: Mike Fenelon <mike.fenelon@microsoft.com>