Document intentional fallthroughs.

Found by: Coverity scan.

grub-core/video/cirrus.c

@@ -440,6 +440,7 @@ grub_video_cirrus_setup (unsigned int width, unsigned int height,
     case 32:
       framebuffer.mode_info.reserved_mask_size = 8;
       framebuffer.mode_info.reserved_field_pos = 24;
+      /* Fallthrough.  */
 
     case 24:
       framebuffer.mode_info.red_mask_size = 8;