XNAT: Fix Qt5 build error related to ambiguous call to QStringRef::compare

This commit fixes the following build error:

/path/to/CTK/Libs/XNAT/Core/ctkXnatResourceCatalogXmlParser.cpp: In member function ‘void ctkXnatResourceCatalogXmlParser::parseXml(QList<QMap<QString, QVariant> >&)’:
/path/to/CTK/Libs/XNAT/Core/ctkXnatResourceCatalogXmlParser.cpp:60:44: error: call of overloaded ‘compare(const char [6])’ is ambiguous
     if (d->xmlReader.name().compare("entry") == 0)
                                            ^

Libs/XNAT/Core/ctkXnatResourceCatalogXmlParser.cpp

@@ -57,7 +57,7 @@ void ctkXnatResourceCatalogXmlParser::parseXml(QList<QVariantMap>& result)
 
   while (!d->xmlReader.atEnd())
   {
-    if (d->xmlReader.name().compare("entry") == 0)
+    if (d->xmlReader.name().compare(QLatin1String("entry")) == 0)
     {
       QVariantMap map;
       QXmlStreamAttributes attributes = d->xmlReader.attributes();