OS X: deployqtHelper: never deploy anyting twice

There are targets that have a "depends" on "deployqt",
and since the Makefile doesn't know any specifics, the deployqtHelper
script is run twice when calling e.g.
make deployqt
make codesign
make dmg

That results in the "make dmg" calling the helper script again, which
overwrites for example the previously signed designer plugins with
the unsigned original again, leading to code signature verification
failure. This became only apparent since OS X 10.9 changed to always
do deep verification of signatures.

Change-Id: I36ec6de1a7d184800c59a3db47e01afedc0e6879
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>