Linux deployment: Only add RPATH to Qt if necessary

Don't try to add an RPATH to Qt if the library/executable
does not link to Qt. Chances are, that the reserved space
for the RPATH is too short in that case.

Change-Id: Ie8d50ede43d19236611fe56667995e58606062db
Reviewed-by: Eike Ziller <eike.ziller@qt.io>