Include SDL_events.h in SDL_main.h

Applications shouldn't need to include it separately if they want to process events using the main callbacks.

Fixes https://github.com/libsdl-org/SDL/issues/8746

include/SDL3/SDL_main.h

@@ -23,6 +23,7 @@
 #define SDL_main_h_
 
 #include <SDL3/SDL_stdinc.h>
+#include <SDL3/SDL_events.h>
 
 /*
  * For details on how SDL_main works, and how to use it, please refer to:
@@ -157,10 +158,9 @@
 extern "C" {
 #endif
 
-union SDL_Event;
 typedef int (SDLCALL *SDL_AppInit_func)(int argc, char *argv[]);
 typedef int (SDLCALL *SDL_AppIterate_func)(void);
-typedef int (SDLCALL *SDL_AppEvent_func)(const union SDL_Event *event);
+typedef int (SDLCALL *SDL_AppEvent_func)(const SDL_Event *event);
 typedef void (SDLCALL *SDL_AppQuit_func)(void);
 
 /**