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);

PR-URL: https://github.com/nodejs/node/pull/11197


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>