replaced mach_absolute_time()

- Addresses issue #15115
- mach_absolute_time can be misused for fingerprinting. This is bad.
- Apple docs prefer clock_gettime_nsec_np(CLOCK_UPTIME_RAW) in time.h
- Since HAVE_NANOSLEEP is defined, time.h is included

src/timer/unix/SDL_systimer.c

@@ -101,7 +101,8 @@ Uint64 SDL_GetPerformanceCounter(void)
         ticks *= SDL_NS_PER_SECOND;
         ticks += now.tv_nsec;
 #elif defined(SDL_PLATFORM_APPLE)
-        ticks = mach_absolute_time();
+        // With HAVE_NANOSLEEP defined, time.h is included
+        ticks = clock_gettime_nsec_np(CLOCK_UPTIME_RAW);
 #else
         SDL_assert(false);
         ticks = 0;