Fix clang warnings

../src/native-state-drm.cpp:334:20: error: cannot pass object of non-trivial type 'std::__cxx11::basic_string<char>' through variadic function; call will abort at runtime [-Wnon-pod-varargs]
                   dev_path);
                   ^
1 error generated.

Signed-off-by: Khem Raj <raj.khem@gmail.com>