GCC 4.8 warning fix.

Changes introduced with commit c16d3d6 cause gcc 4.8 to fail:

    [  0%] Building CXX object libOpenImageIO/CMakeFiles/OpenImageIO.dir/exif.cpp.o
    In file included from /home/mariusz/repos/oiio/src/include/strutil.h:53:0,
                     from /home/mariusz/repos/oiio/src/include/ustring.h:137,
                     from /home/mariusz/repos/oiio/src/include/paramlist.h:52,
                     from /home/mariusz/repos/oiio/src/include/imageio.h:59,
                     from /home/mariusz/repos/oiio/src/libOpenImageIO/exif.cpp:74:
    /home/mariusz/repos/oiio/src/include/tinyformat.h: In function ‘void tinyformat::formatValue(std::ostream&, const char*, const char*, const T&)’:
    /home/mariusz/repos/oiio/src/include/tinyformat.h:261:77: error: typedef ‘DummyType’ locally defined but not used [-Werror=unused-local-typedefs]
         typedef typename detail::is_wchar<T>::tinyformat_wchar_is_not_supported DummyType;
                                                                                 ^
    cc1plus: all warnings being treated as errors
    make[3]: *** [libOpenImageIO/CMakeFiles/OpenImageIO.dir/exif.cpp.o] Error 1

[Relevant Chromium issue](https://codereview.chromium.org/10449006/)