Fix a fallthrough error in GCC 7+

GCC 7 and higher turn a previous warning into an error for implicit
fallthrough. Let's hint the compiler that this is intentional here.

Signed-off-by: Kai Krakow <kai@kaishome.de>

src/bees.cc

@@ -635,7 +635,7 @@ bees_main(int argc, char *argv[])
 				crucible::set_relative_path("");
 				break;
 			case 'h':
-				do_cmd_help(argv);
+				do_cmd_help(argv); // fallthrough
 			default:
 				return 2;
 		}