CppTools: Provide whether a sub project is selected for building

Analyzers can use this to rule out irrelevant (sub) projects.

Change-Id: I5a9eb66f06156f0619dc639f6f4080a8fc761ee6
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/plugins/cpptools/cppprojects.cpp

@@ -49,6 +49,7 @@ ProjectPart::ProjectPart()
     , languageExtensions(NoExtensions)
     , qtVersion(UnknownQt)
     , warningFlags(ProjectExplorer::ToolChain::WarningsDefault)
+    , selectedForBuilding(true)
 {
 }