CMake: Cache buildtargets in builddirmanager

No need to have the builddirreaders to so.

Change-Id: Ia7e38f6483b29d6a4f4b65baacc17bf184a3cb34
Task-number: QTCREATORBUG-17359
Reviewed-by: Florian Apolloner <florian@apolloner.eu>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/servermodereader.h

@@ -147,7 +147,7 @@ private:
     QList<ProjectExplorer::FileNode *> m_cmakeInputsFileNodes;
 
     QList<Project *> m_projects;
-    QList<Target *> m_targets;
+    mutable QList<Target *> m_targets;
     QList<FileGroup *> m_fileGroups;
 };