Stop message skip on unseen text

This makes System3-sdl2 stop message skip when it sees text that has not
been read by the player. Seen / unseen flags are stored in MSGSKIP.DAT
file in the save folder.

src/linux/nact_linux.cpp

@@ -24,6 +24,10 @@ void NACT::output_console(const char *format, ...)
 #endif
 }
 
+void NACT::set_skip_menu_state(bool enabled, bool checked)
+{
+}
+
 bool NACT::handle_platform_event(const SDL_Event& e)
 {
 	return false;