Fix for building without libpcap and extcap.

(A further fix should be "don't put the "Capture" section into the
welcome screen if we have neither libpcap nor extcap".)

Change-Id: I83e65e6dc31040292af7fe88ccd73e485613c76f
Reviewed-on: https://code.wireshark.org/review/22634
Reviewed-by: Guy Harris <guy@alum.mit.edu>

ui/qt/interface_toolbar.cpp

@@ -917,6 +917,7 @@ void InterfaceToolbar::updateWidgets()
 
 void InterfaceToolbar::interfaceListChanged()
 {
+#ifdef HAVE_LIBPCAP
     const QString &selected_ifname = ui->interfacesComboBox->currentText();
     bool keep_selected = false;
 
@@ -950,6 +951,7 @@ void InterfaceToolbar::interfaceListChanged()
     }
 
     updateWidgets();
+#endif
 }
 
 void InterfaceToolbar::on_interfacesComboBox_currentIndexChanged(const QString &ifname)