r300: Corrected an error in r300_vertprog.c.

Cannot use _mesa_warning here because we don't have a context pointer; perhaps
R300 specific warning and error functions are needed.

src/mesa/drivers/dri/r300/r300_vertprog.c

@@ -255,7 +255,7 @@ static unsigned long t_src_index(struct r300_vertex_program *vp, struct prog_src
 		return vp->inputs[src->Index];
 	}else{
 		if (src->Index < 0) {
-			_mesa_warning ("negative offsets for indirect addressing do not work.\n");
+			fprintf (stderr, "negative offsets for indirect addressing do not work.\n");
 			return 0;
 		}
 		return src->Index;