caca: Fix a warning about unsigned.

modules/video_output/caca.c

@@ -336,8 +336,8 @@ static void Refresh(vout_display_t *vd)
     caca_refresh_display(sys->dp);
 
     /* */
-    const int width  = caca_get_display_width(sys->dp);
-    const int height = caca_get_display_height(sys->dp);
+    const unsigned width  = caca_get_display_width(sys->dp);
+    const unsigned height = caca_get_display_height(sys->dp);
 
     if (width  != vd->cfg->display.width ||
         height != vd->cfg->display.height)