fix(telemetry): use local link-type constants in followsFrom linkedSpan

The followsFrom linkedSpan overload referenced attr::linkType /
attr_val::followsFrom, which don't exist in SpanNames.h — the other two
link sites already use the local kLinkTypeKey / kLinkTypeFollowsFrom
constants. Unify the third site. Fixes arm64 build (-Werror).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/libxrpl/telemetry/SpanGuard.cpp

@@ -317,8 +317,7 @@ SpanGuard::hashSpan(
                 std::make_unique<Impl>(tracer->StartSpan(
                     std::string(name),
                     {},
-                    {{linkSpan->GetContext(),
-                      {{std::string(attr::linkType), std::string(attr_val::followsFrom)}}}},
+                    {{linkSpan->GetContext(), {{kLinkTypeKey, kLinkTypeFollowsFrom}}}},
                     opts)));
         }
     }