fixed bug when omp was disabled by default

src/mandelbrot.c

@@ -352,7 +352,7 @@ main(int argc, char **argv)
     struct arguments args = {
         NULL, 
         640, 480, 
-        250, 1, 0,
+        250, 1, 1,
         0,-2.0, 1.0, -1.5, 1.5, 1.0,
     };
     argp_parse(&argp, argc, argv, 0, 0, &args);