luci-base: dispatcher.uc: reintroduce support for 'absent' fs depends

The ability to require certain paths to be absent for a menu entry to show
up was lost during the ucode conversion.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/ucode/dispatcher.uc

@@ -183,6 +183,10 @@ function check_fs_depends(spec) {
 			if (fstat?.type != 'file')
 				return false;
 		}
+		else if (kind == 'absent') {
+			if (stat(path) != null)
+				return false;
+		}
 	}
 
 	return true;