wfc.h as an include

[nsmryan]

I've noticed that if I have wfc.h's implementation in one file, and want to use as a header in another, there are a number of functions without prototypes.

For example, I modified the header to declare some prototypes:

int wfc_export(struct wfc *wfc, const char *filename);
void wfc_destroy(struct wfc *wfc);
struct wfc_image *wfc_output_image(struct wfc *wfc);
void wfc_img_destroy(struct wfc_image *image);

I might be able to come up with a PR for this if you would accept it. I would include wrapping in WFC_USE_STB for functions not present without that flag.