Import SCons.Errors to fix error at line 35

Change-Id: I683b6602afcc26984be146d800559532df729675
Signed-off-by: Benjamin Cabé <kartben@gmail.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/1158Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>

extlibs/boost/SConscript

 import os, string, sys, subprocess
+import SCons.Errors
 
 Import('env')