Fix navreceiver to make DME work with ILS signals.

Property "dme-in-range" should also work with ILS signals, which include
a DME. The nav database (dme_list) doesn't have separate entries for
"ILS-DME" (unlike for "VOR-DME"), so searching a DME by frequency didn't
work for ILS.
=> "instrumentation/nav[*]/dme-in-range" can now be used to enable/disable
cockpit DME displays.

src/Instrumentation/navradio.cxx

@@ -940,6 +940,10 @@ void FGNavRadio::search()
   _navaid = nav;
   string identBuffer(4, ' ');
   if (nav) {
+    // use ILS signals as DME, otherwise search by frequency
+    if (nav->type()==FGPositioned::ILS)
+        _dme = nav;
+    else
         _dme = globals->get_dmelist()->findByFreq(freq, pos);
     
     nav_id_node->setStringValue(nav->get_ident());