Fix abs(long) warnings in gd.c

I'm fixing this using (int) casts instead of labs() because this is
what upstream GD does.

Author: nikic

ext/gd/libgd/gd.c

@@ -1256,7 +1256,7 @@ void gdImageAALine (gdImagePtr im, int x1, int y1, int x2, int y2, int col)
 	if (dx == 0 && dy == 0) {
 		return;
 	}
-	if (abs(dx) > abs(dy)) {
+	if (abs((int)dx) > abs((int)dy)) {
 		if (dx < 0) {
 			tmp = x1;
 			x1 = x2;