refactor: keep the gs_usb option out of non-Windows builds

The radio button was only disabled elsewhere, which advertised a connection
type that platform can never offer. Hide it instead, and guard the gs_usb.h
include so nothing outside Windows reaches for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XZnuZJ7zc3e8hk6C8bGDZN

connections/canconfactory.cpp

@@ -7,7 +7,9 @@
 #include "lawicel_serial.h"
 #include "canserver.h"
 #include "canlogserver.h"
+#ifdef Q_OS_WIN
 #include "gs_usb.h"
+#endif
 
 using namespace CANCon;