qttool: depends on network-support, but should only be optional

Building Qt without network-support fails as QtTools depends on it. 

For our custom CMake build, we just need QtLinguist from Qttools, but excluded all other QtTools-components from build. As far as I can see "QtAssistant", "QtDesigner" have a hard dependency on Network and "qtdiag" can handle network as optional feature already. 

When disabling "designer" and "assistant" there should be no hard dependency to network. Similar to the way it's done with SQL-support.