Do not use flat_set anymore gives compilation issues.

Not worth the time solving the issue without having prove of a clear benefit of
using flat_set over unordered_set.

pglablib/catalog/PgKeywordList.h

@@ -27,6 +27,7 @@ public:
 
 	//bool operator<(const QString &s) const { return m_keyword < s; }
 	bool operator<(const Keyword &kw) const { return m_keyword < kw.m_keyword; }
+	bool operator==(const Keyword &kw) const { return m_keyword == kw.m_keyword; }
 private:
 	QString m_keyword;
 	int16_t m_category;