CMake: Implement CMakeTargetNode::buildKey()

This always sets a node id, that is reasonably unique. So use it.

Change-Id: I9b1cb978f0d6edffabee4eafa5ec078a6a6827a6
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/cmakeprojectnodes.cpp

@@ -196,6 +196,11 @@ QString CMakeTargetNode::tooltip() const
     return m_tooltip;
 }
 
+QString CMakeTargetNode::buildKey() const
+{
+    return QString::fromUtf8(id());
+}
+
 bool CMakeTargetNode::supportsAction(ProjectExplorer::ProjectAction action,
                                      const ProjectExplorer::Node *) const
 {