I've seen posts at the wx forum where problems of this kind were caused by the fact that the wx headers were not included before anything else.
So I'm not sure, but you may want to try to reverse the order of the sfml- and wx include in wxSFMLCanvas.hpp:
#include <wx/wx.h>
#include <SFML/Graphics.hpp>
Please let me know if this helps.
Good luck!
Theo.