Seems underscored APIs are not visible on Win32. Anyway even if we fixed
this, this would only be temporary as all these deprecated APIs are only
meant to stay around temporarily, then be actually unavailable.

I hesitated to actually undeprecate the `_gimp_*_is_valid()` (like
making a `gimp_image_is_valid_id()` API for instance) but this is
actually quite similar to just create the GimpImage: if
gimp_image_get_by_id() returns NULL, then the ID was not valid.