add using...

tests/testSerializationSlam.cpp

@@ -614,6 +614,8 @@ static GaussianFactorGraph read(const string& name) {
 /* ************************************************************************* */
 // Read from XML file
 TEST(SubgraphSolver, Solves) {
+  using gtsam::example::planarGraph;
+  
   // Create preconditioner
   SubgraphPreconditioner system;