Fix GCC build with foward struct startup declaration.

Interestingly, not required for successful MSVC build. <shurg>

I suspecte the circular dependency and include-guards around mqtt.h
and startup.h were going to be a problem.

src/sbbs3/mqtt.h

@@ -28,6 +28,8 @@
 #include "link_list.h"
 #include "dllexport.h"
 
+struct startup;
+
 #include <stdarg.h>
 
 #ifdef USE_MOSQUITTO