www/firefox: unbreak after r520537 if gtk3 lacks Wayland support

In file included from Unified_cpp_toolkit_xre0.cpp:56:
toolkit/xre/glxtest.cpp:39:10: fatal error: 'mozilla/widget/mozwayland.h' file not found
#include "mozilla/widget/mozwayland.h"

^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PR: 242790
Reported by: many
Submitted by: Greg V