Make object_path operator/ const

This is a minor change to make operator/ const.  It likely should've
been const initially, but I didn't think of it, and I didn't have a
const example of an input initially.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ifb115a26cce6baa834752ceebdd81dc0d2e0cf14
diff --git a/include/sdbusplus/message/native_types.hpp b/include/sdbusplus/message/native_types.hpp
index df68dbd..e13c295 100644
--- a/include/sdbusplus/message/native_types.hpp
+++ b/include/sdbusplus/message/native_types.hpp
@@ -174,12 +174,12 @@
         return str.substr(0, index);
     }
 
-    string_path_wrapper operator/(const std::string& extId)
+    string_path_wrapper operator/(const std::string& extId) const
     {
         return this->operator/(extId.c_str());
     }
 
-    string_path_wrapper operator/(const char* extId)
+    string_path_wrapper operator/(const char* extId) const
     {
         string_path_wrapper out;
         _cleanup_free_ char* encOut = nullptr;