Fix clang build

Without it I will get:

    src/pvr2wmc.cpp:206:61: error: cannot pass object of non-POD type 'std::vector<CStdString>::iterator' (aka '__normal_iterator<pointer, vector_type>') through variadic
      method; call will abort at runtime [-Wnon-pod-varargs]
                        XBMC->Log(LOG_INFO, "Received message from backend: %s", response);
                                                                                 ^

building on Mac with clang.