Mac: Fix a warning about implicit declaration of SDL_SendKeymapChangedEvent (bug #3167.)

src/video/cocoa/SDL_cocoakeyboard.m

@@ -24,6 +24,7 @@
 
 #include "SDL_cocoavideo.h"
 
+#include "../../events/SDL_events_c.h"
 #include "../../events/SDL_keyboard_c.h"
 #include "../../events/scancodes_darwin.h"