src: fix -Wunused-result compiler warning
Fix a warning that was introduced in commit 67af1ad6 ("src: refactor CopyProperties to remove JS") from a few days ago. This particular change was suggested by me, mea culpa.
Fixes the following warning:
../src/node_contextify.cc:151:13: warning: ignoring return
value of function declared with warn_unused_result attribute
[-Wunused-result]
sandbox_obj->DefineProperty(context, key, *desc);