Make the extension check for graphics import case insensitive

The regex for the file extension that matches against all case
possibilities is only built on Linux.

Fixes: lp:1842180
* https://bugs.launchpad.net/kicad/+bug/1842180

pcbnew/import_gfx/graphics_import_mgr.cpp

@@ -72,7 +72,7 @@ std::unique_ptr<GRAPHICS_IMPORT_PLUGIN> GRAPHICS_IMPORT_MGR::GetPluginByExt(
 
         for( const auto& fileExt : fileExtensions )
         {
-            wxRegEx extensions( fileExt, wxRE_BASIC );
+            wxRegEx extensions( fileExt, wxRE_ICASE );
 
             if( extensions.Matches( aExtension ) )
                 return plugin;