make-ring: fix dring path for launching in gdb

When launching with the --debug option, the path for dring was
wrong, it was the old path before the location of the binary was
changed.

Reviewed-by: Alexandre Viau <alexandre.viau@savoirfairelinux.com>
Change-Id: I5d319cd069bf4c9a95ffaa6b52470f612653679c