certificate-service: fix clang-tidy warnings

```
../redfish-core/lib/certificate_service.hpp:154:32: error: 'i' is an unsafe pointer used for buffer access [-Werror,-Wunsafe-buffer-usage]
    std::string_view::iterator i = value.begin();
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I6b6fb17d58bb7a3bd861aaf020540586490693da
diff --git a/redfish-core/lib/certificate_service.hpp b/redfish-core/lib/certificate_service.hpp
index 3cb6a3f..dcbff2d 100644
--- a/redfish-core/lib/certificate_service.hpp
+++ b/redfish-core/lib/certificate_service.hpp
@@ -157,18 +157,18 @@
         std::string_view::iterator tokenBegin = i;
         while (i != value.end() && *i != '=')
         {
-            ++i;
+            std::advance(i, 1);
         }
         if (i == value.end())
         {
             break;
         }
         std::string_view key(tokenBegin, static_cast<size_t>(i - tokenBegin));
-        ++i;
+        std::advance(i, 1);
         tokenBegin = i;
         while (i != value.end() && *i != ',')
         {
-            ++i;
+            std::advance(i, 1);
         }
         std::string_view val(tokenBegin, static_cast<size_t>(i - tokenBegin));
         if (key == "L")
@@ -198,7 +198,7 @@
         // skip comma character
         if (i != value.end())
         {
-            ++i;
+            std::advance(i, 1);
         }
     }
 }