fixed BoundaryHandling test case

tests/pde/BoundaryTest.cpp

@@ -48,7 +48,7 @@
 
 #include <cmath>
 
-using namespace walberla;
+namespace walberla {
 
 
 
@@ -437,3 +437,9 @@ int main( int argc, char** argv )
    logging::Logging::printFooterOnStream();
    return EXIT_SUCCESS;
 }
+}
+
+int main( int argc, char* argv[] )
+{
+  return walberla::main( argc, argv );
+}