Include cstring header on image_png.hpp

Compilation was failing with the "enable png" flag. This was happening
because the memcpy function was being called without the inclusion of
cstring, which declares it. This fixes it.

source/paint/detail/image_png.hpp

@@ -2,6 +2,7 @@
 #define NANA_PAINT_DETAIL_IMAGE_PNG_HPP
 
 #include "image_pixbuf.hpp"
+#include <cstring>
 
 //Separate the libpng from the package that system provides.
 #if defined(NANA_LIBPNG)