PMsrHandler: escape '?' to avoid trigraph warning in date placeholder

The fallback date placeholder string contained the sequence '??-', which
the compiler interprets as a trigraph for '~' (-Wtrigraphs warning).
Escape the question marks (\?) so the literal string is unchanged while
the warning is silenced.

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

src/classes/PMsrHandler.cpp

@@ -4993,7 +4993,7 @@ Bool_t PMsrHandler::HandleStatisticEntry(PMsrLines &lines)
       if (status == 2) {
         fStatistic.fDate = TString(date)+TString(", ")+TString(time);
       } else {
-        fStatistic.fDate = TString("????-??-??, ??:??:??");
+        fStatistic.fDate = TString("\?\?\?\?-\?\?-\?\?, \?\?:\?\?:\?\?");
       }
     }
     // extract chisq