CMake: Fix GCC warnings

warning: control reaches end of non-void function [-Wreturn-type]

https://stackoverflow.com/q/33607284/764870

Change-Id: Ia8d0a8d42d5db786c6800ab91a1ef790fe2387f1
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cmakeprojectmanager/cmaketool.cpp

@@ -73,6 +73,8 @@ static QString readerTypeToString(const CMakeTool::ReaderType &type)
     switch (type) {
     case CMakeTool::FileApi:
         return QString(READER_TYPE_FILEAPI);
+    default:
+        return QString();
     }
 }