CMake: Clear files when cleaning CMakeBuildTargets

Change-Id: I16fefae569f807776e422b4b662d5b42d1230df2
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/cmakeprojectmanager/cmakeproject.cpp

@@ -496,6 +496,7 @@ void CMakeBuildTarget::clear()
     includeFiles.clear();
     compilerOptions.clear();
     defines.clear();
+    files.clear();
 }
 
 } // namespace CMakeProjectManager