Class 'NullLineEdit' lacks Q_OBJECT macro

Qt Creator and lupdate are giving warnings about this class lacking the
Q_OBJECT macro. Although it seems to not be needed, adding it will silence
the warnings without drawbacks.

See issue #1740.

src/AddRecordDialog.cpp

@@ -12,6 +12,7 @@
 #include <QMenu>
 
 class NullLineEdit: public QLineEdit {
+    Q_OBJECT
 private:
     bool m_isNull;
 
@@ -338,3 +339,5 @@ void AddRecordDialog::on_buttonBox_clicked(QAbstractButton* button)
             populateFields();
     }
 }
+
+#include "AddRecordDialog.moc"