fix build under gcc

after my commit feb92a5f
"inline some strings, they don't need to be in a shared header file"

Change-Id: I60a6fa11d56f3d4648dbedeca062ab27206f2a02

chart2/source/model/main/CartesianCoordinateSystem.cxx

@@ -64,13 +64,13 @@ CartesianCoordinateSystem::~CartesianCoordinateSystem()
 OUString SAL_CALL CartesianCoordinateSystem::getCoordinateSystemType()
     throw (RuntimeException, std::exception)
 {
-    return CHART2_COOSYSTEM_CARTESIAN_SERVICE_NAME;
+    return OUString(CHART2_COOSYSTEM_CARTESIAN_SERVICE_NAME);
 }
 
 OUString SAL_CALL CartesianCoordinateSystem::getViewServiceName()
     throw (RuntimeException, std::exception)
 {
-    return CHART2_COOSYSTEM_CARTESIAN_VIEW_SERVICE_NAME;
+    return OUString(CHART2_COOSYSTEM_CARTESIAN_VIEW_SERVICE_NAME);
 }
 
 // ____ XCloneable ____