Corrected type conversion in C file

On my Mac (10.9.1) `go run <any_example>` it shown warnings:

```sh
src/github.com/saratovsource/go-opencv/opencv/opencv.c:26:21: warning: passing 'schar *' (aka 'signed char *') to parameter of type 'char *' converts between pointers to integer types with different sign [-Wpointer-sign]
src/github.com/saratovsource/go-opencv/opencv/highgui.go:37:38: note: passing argument to parameter 'p0' here
src/github.com/saratovsource/go-opencv/opencv/opencv.c:26:36: warning: passing 'schar *' (aka 'signed char *') to parameter of type 'char *' converts between pointers to integer types with different sign [-Wpointer-sign]
src/github.com/saratovsource/go-opencv/opencv/highgui.go:37:48: note: passing argument to parameter 'p1' here
src/github.com/saratovsource/go-opencv/opencv/opencv.c:62:18: warning: passing 'schar *' (aka 'signed char *') to parameter of type 'char *' converts between pointers to integer types with different sign [-Wpointer-sign]
src/github.com/saratovsource/go-opencv/opencv/highgui.go:39:35: note: passing argument to parameter 'p0' here
```

I tried to make the same types in C file and warnings stopped output.
Can you test it in your environments?