Add more const const

Found by clang-tidy.

Change-Id: I4faa8135052690f59020779408894eb951e707e5
diff --git a/src/path_parser.hpp b/src/path_parser.hpp
index eb34033..fca760d 100644
--- a/src/path_parser.hpp
+++ b/src/path_parser.hpp
@@ -433,7 +433,7 @@
         return true;
     }
 
-} writableLeafPath;
+} const writableLeafPath;
 
 struct RpcActionPath : x3::parser<RpcActionPath> {
     using attribute_type = dataPath_;