export private module's deps as run deps of the public module

this is necessary for:
- generating -rpath-link arguments when link_prl is not used. link_prl
  is enabled by default, so this has no effect on most projects.
- deployment purposes, which is hypothetical as of now.

Change-Id: I9e629f3eef93c4edf12efc016ecc27dbe2186d61
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>