LibCore: Implement operator less-than for Core::DateTime

that just compares their timestamps.

Userland/Libraries/LibCore/DateTime.h

@@ -59,6 +59,7 @@ public:
     // FIXME: This should be replaced with a proper comparison
     //        operator when we get the equivalent of strptime
     bool is_before(const String&) const;
+    bool operator<(const DateTime& other) const { return m_timestamp < other.m_timestamp; }
 
 private:
     time_t m_timestamp { 0 };