GerbView #2124 : Alternative drill file extension .txt

'.txt' is a common file extension for drill files.  This Merge
Request adds it to the list of extensions used in the File Open dialog
for Excellon Drill files.

Fixes issue https://gitlab.com/kicad/code/kicad/-/issues/2124

common/wildcards_and_files_ext.cpp

@@ -342,7 +342,7 @@ wxString ComponentFileWildcard()
 // Wildcard for reports and fabrication documents
 wxString DrillFileWildcard()
 {
-    return _( "Drill files" ) + AddFileExtListToFilter( { "drl", "nc", "xnc" } );
+    return _( "Drill files" ) + AddFileExtListToFilter( { "drl", "nc", "xnc", "txt" } );
 }