Fix warning for Android NDK compiler: "function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]"

https://stackoverflow.com/questions/42125/warning-error-function-declaration-isnt-a-prototype
In C int foo() and int foo(void) are different functions. int foo() accepts an arbitrary number of arguments, while int foo(void) accepts 0 arguments. In C++ they mean the same thing.

src/timer/windows/SDL_systimer.c

@@ -31,7 +31,7 @@ static void SDL_CleanupWaitableTimer(void *timer)
     CloseHandle(timer);
 }
 
-HANDLE SDL_GetWaitableTimer()
+HANDLE SDL_GetWaitableTimer(void)
 {
     static SDL_TLSID TLS_timer_handle;
     HANDLE timer;